From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001 From: root Date: Fri, 25 Dec 2015 04:40:36 +0000 Subject: initial_commit --- Documentation/DocBook/kernel-locking.tmpl | 2146 +++++++++++++++++++++++++++++ 1 file changed, 2146 insertions(+) create mode 100644 Documentation/DocBook/kernel-locking.tmpl (limited to 'Documentation/DocBook/kernel-locking.tmpl') diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl new file mode 100644 index 00000000..67e7ab41 --- /dev/null +++ b/Documentation/DocBook/kernel-locking.tmpl @@ -0,0 +1,2146 @@ + + + + + + Unreliable Guide To Locking + + + + Rusty + Russell + +
+ rusty@rustcorp.com.au +
+ + + + + + 2003 + Rusty Russell + + + + + This documentation is free software; you can redistribute + it and/or modify it under the terms of the GNU General Public + License as published by the Free Software Foundation; either + version 2 of the License, or (at your option) any later + version. + + + + This program is distributed in the hope that it will be + useful, but WITHOUT ANY WARRANTY; without even the implied + warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. + See the GNU General Public License for more details. + + + + You should have received a copy of the GNU General Public + License along with this program; if not, write to the Free + Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, + MA 02111-1307 USA + + + + For more details see the file COPYING in the source + distribution of Linux. + + + + + + + Introduction + + Welcome, to Rusty's Remarkably Unreliable Guide to Kernel + Locking issues. This document describes the locking systems in + the Linux Kernel in 2.6. + + + With the wide availability of HyperThreading, and preemption in the Linux + Kernel, everyone hacking on the kernel needs to know the + fundamentals of concurrency and locking for + SMP. + + + + + The Problem With Concurrency + + (Skip this if you know what a Race Condition is). + + + In a normal program, you can increment a counter like so: + + + very_important_count++; + + + + This is what they would expect to happen: + + + + Expected Results + + + + + + Instance 1 + Instance 2 + + + + + + read very_important_count (5) + + + + add 1 (6) + + + + write very_important_count (6) + + + + + read very_important_count (6) + + + + add 1 (7) + + + + write very_important_count (7) + + + + +
+ + + This is what might happen: + + + + Possible Results + + + + + Instance 1 + Instance 2 + + + + + + read very_important_count (5) + + + + + read very_important_count (5) + + + add 1 (6) + + + + + add 1 (6) + + + write very_important_count (6) + + + + + write very_important_count (6) + + + +
+ + + Race Conditions and Critical Regions + + This overlap, where the result depends on the + relative timing of multiple tasks, is called a race condition. + The piece of code containing the concurrency issue is called a + critical region. And especially since Linux starting running + on SMP machines, they became one of the major issues in kernel + design and implementation. + + + Preemption can have the same effect, even if there is only one + CPU: by preempting one task during the critical region, we have + exactly the same race condition. In this case the thread which + preempts might run the critical region itself. + + + The solution is to recognize when these simultaneous accesses + occur, and use locks to make sure that only one instance can + enter the critical region at any time. There are many + friendly primitives in the Linux kernel to help you do this. + And then there are the unfriendly primitives, but I'll pretend + they don't exist. + + + + + + Locking in the Linux Kernel + + + If I could give you one piece of advice: never sleep with anyone + crazier than yourself. But if I had to give you advice on + locking: keep it simple. + + + + Be reluctant to introduce new locks. + + + + Strangely enough, this last one is the exact reverse of my advice when + you have slept with someone crazier than yourself. + And you should think about getting a big dog. + + + + Two Main Types of Kernel Locks: Spinlocks and Mutexes + + + There are two main types of kernel locks. The fundamental type + is the spinlock + (include/asm/spinlock.h), + which is a very simple single-holder lock: if you can't get the + spinlock, you keep trying (spinning) until you can. Spinlocks are + very small and fast, and can be used anywhere. + + + The second type is a mutex + (include/linux/mutex.h): it + is like a spinlock, but you may block holding a mutex. + If you can't lock a mutex, your task will suspend itself, and be woken + up when the mutex is released. This means the CPU can do something + else while you are waiting. There are many cases when you simply + can't sleep (see ), and so have to + use a spinlock instead. + + + Neither type of lock is recursive: see + . + + + + + Locks and Uniprocessor Kernels + + + For kernels compiled without CONFIG_SMP, and + without CONFIG_PREEMPT spinlocks do not exist at + all. This is an excellent design decision: when no-one else can + run at the same time, there is no reason to have a lock. + + + + If the kernel is compiled without CONFIG_SMP, + but CONFIG_PREEMPT is set, then spinlocks + simply disable preemption, which is sufficient to prevent any + races. For most purposes, we can think of preemption as + equivalent to SMP, and not worry about it separately. + + + + You should always test your locking code with CONFIG_SMP + and CONFIG_PREEMPT enabled, even if you don't have an SMP test box, because it + will still catch some kinds of locking bugs. + + + + Mutexes still exist, because they are required for + synchronization between user + contexts, as we will see below. + + + + + Locking Only In User Context + + + If you have a data structure which is only ever accessed from + user context, then you can use a simple mutex + (include/linux/mutex.h) to protect it. This + is the most trivial case: you initialize the mutex. Then you can + call mutex_lock_interruptible() to grab the mutex, + and mutex_unlock() to release it. There is also a + mutex_lock(), which should be avoided, because it + will not return if a signal is received. + + + + Example: net/netfilter/nf_sockopt.c allows + registration of new setsockopt() and + getsockopt() calls, with + nf_register_sockopt(). Registration and + de-registration are only done on module load and unload (and boot + time, where there is no concurrency), and the list of registrations + is only consulted for an unknown setsockopt() + or getsockopt() system call. The + nf_sockopt_mutex is perfect to protect this, + especially since the setsockopt and getsockopt calls may well + sleep. + + + + + Locking Between User Context and Softirqs + + + If a softirq shares + data with user context, you have two problems. Firstly, the current + user context can be interrupted by a softirq, and secondly, the + critical region could be entered from another CPU. This is where + spin_lock_bh() + (include/linux/spinlock.h) is + used. It disables softirqs on that CPU, then grabs the lock. + spin_unlock_bh() does the reverse. (The + '_bh' suffix is a historical reference to "Bottom Halves", the + old name for software interrupts. It should really be + called spin_lock_softirq()' in a perfect world). + + + + Note that you can also use spin_lock_irq() + or spin_lock_irqsave() here, which stop + hardware interrupts as well: see . + + + + This works perfectly for UP + as well: the spin lock vanishes, and this macro + simply becomes local_bh_disable() + (include/linux/interrupt.h), which + protects you from the softirq being run. + + + + + Locking Between User Context and Tasklets + + + This is exactly the same as above, because tasklets are actually run + from a softirq. + + + + + Locking Between User Context and Timers + + + This, too, is exactly the same as above, because timers are actually run from + a softirq. From a locking point of view, tasklets and timers + are identical. + + + + + Locking Between Tasklets/Timers + + + Sometimes a tasklet or timer might want to share data with + another tasklet or timer. + + + + The Same Tasklet/Timer + + Since a tasklet is never run on two CPUs at once, you don't + need to worry about your tasklet being reentrant (running + twice at once), even on SMP. + + + + + Different Tasklets/Timers + + If another tasklet/timer wants + to share data with your tasklet or timer , you will both need to use + spin_lock() and + spin_unlock() calls. + spin_lock_bh() is + unnecessary here, as you are already in a tasklet, and + none will be run on the same CPU. + + + + + + Locking Between Softirqs + + + Often a softirq might + want to share data with itself or a tasklet/timer. + + + + The Same Softirq + + + The same softirq can run on the other CPUs: you can use a + per-CPU array (see ) for better + performance. If you're going so far as to use a softirq, + you probably care about scalable performance enough + to justify the extra complexity. + + + + You'll need to use spin_lock() and + spin_unlock() for shared data. + + + + + Different Softirqs + + + You'll need to use spin_lock() and + spin_unlock() for shared data, whether it + be a timer, tasklet, different softirq or the same or another + softirq: any of them could be running on a different CPU. + + + + + + + Hard IRQ Context + + + Hardware interrupts usually communicate with a + tasklet or softirq. Frequently this involves putting work in a + queue, which the softirq will take out. + + + + Locking Between Hard IRQ and Softirqs/Tasklets + + + If a hardware irq handler shares data with a softirq, you have + two concerns. Firstly, the softirq processing can be + interrupted by a hardware interrupt, and secondly, the + critical region could be entered by a hardware interrupt on + another CPU. This is where spin_lock_irq() is + used. It is defined to disable interrupts on that cpu, then grab + the lock. spin_unlock_irq() does the reverse. + + + + The irq handler does not to use + spin_lock_irq(), because the softirq cannot + run while the irq handler is running: it can use + spin_lock(), which is slightly faster. The + only exception would be if a different hardware irq handler uses + the same lock: spin_lock_irq() will stop + that from interrupting us. + + + + This works perfectly for UP as well: the spin lock vanishes, + and this macro simply becomes local_irq_disable() + (include/asm/smp.h), which + protects you from the softirq/tasklet/BH being run. + + + + spin_lock_irqsave() + (include/linux/spinlock.h) is a variant + which saves whether interrupts were on or off in a flags word, + which is passed to spin_unlock_irqrestore(). This + means that the same code can be used inside an hard irq handler (where + interrupts are already off) and in softirqs (where the irq + disabling is required). + + + + Note that softirqs (and hence tasklets and timers) are run on + return from hardware interrupts, so + spin_lock_irq() also stops these. In that + sense, spin_lock_irqsave() is the most + general and powerful locking function. + + + + + Locking Between Two Hard IRQ Handlers + + It is rare to have to share data between two IRQ handlers, but + if you do, spin_lock_irqsave() should be + used: it is architecture-specific whether all interrupts are + disabled inside irq handlers themselves. + + + + + + + Cheat Sheet For Locking + + Pete Zaitcev gives the following summary: + + + + + If you are in a process context (any syscall) and want to + lock other process out, use a mutex. You can take a mutex + and sleep (copy_from_user*( or + kmalloc(x,GFP_KERNEL)). + + + + + Otherwise (== data can be touched in an interrupt), use + spin_lock_irqsave() and + spin_unlock_irqrestore(). + + + + + Avoid holding spinlock for more than 5 lines of code and + across any function call (except accessors like + readb). + + + + + + Table of Minimum Requirements + + The following table lists the minimum + locking requirements between various contexts. In some cases, + the same context can only be running on one CPU at a time, so + no locking is required for that context (eg. a particular + thread can only run on one CPU at a time, but if it needs + shares data with another thread, locking is required). + + + Remember the advice above: you can always use + spin_lock_irqsave(), which is a superset + of all other spinlock primitives. + + + +Table of Locking Requirements + + + + + +IRQ Handler A +IRQ Handler B +Softirq A +Softirq B +Tasklet A +Tasklet B +Timer A +Timer B +User Context A +User Context B + + + +IRQ Handler A +None + + + +IRQ Handler B +SLIS +None + + + +Softirq A +SLI +SLI +SL + + + +Softirq B +SLI +SLI +SL +SL + + + +Tasklet A +SLI +SLI +SL +SL +None + + + +Tasklet B +SLI +SLI +SL +SL +SL +None + + + +Timer A +SLI +SLI +SL +SL +SL +SL +None + + + +Timer B +SLI +SLI +SL +SL +SL +SL +SL +None + + + +User Context A +SLI +SLI +SLBH +SLBH +SLBH +SLBH +SLBH +SLBH +None + + + +User Context B +SLI +SLI +SLBH +SLBH +SLBH +SLBH +SLBH +SLBH +MLI +None + + + + +
+ + +Legend for Locking Requirements Table + + + + +SLIS +spin_lock_irqsave + + +SLI +spin_lock_irq + + +SL +spin_lock + + +SLBH +spin_lock_bh + + +MLI +mutex_lock_interruptible + + + + +
+ + + + + + The trylock Functions + + There are functions that try to acquire a lock only once and immediately + return a value telling about success or failure to acquire the lock. + They can be used if you need no access to the data protected with the lock + when some other thread is holding the lock. You should acquire the lock + later if you then need access to the data protected with the lock. + + + + spin_trylock() does not spin but returns non-zero if + it acquires the spinlock on the first try or 0 if not. This function can + be used in all contexts like spin_lock: you must have + disabled the contexts that might interrupt you and acquire the spin lock. + + + + mutex_trylock() does not suspend your task + but returns non-zero if it could lock the mutex on the first try + or 0 if not. This function cannot be safely used in hardware or software + interrupt contexts despite not sleeping. + + + + + Common Examples + +Let's step through a simple example: a cache of number to name +mappings. The cache keeps a count of how often each of the objects is +used, and when it gets full, throws out the least used one. + + + + + All In User Context + +For our first example, we assume that all operations are in user +context (ie. from system calls), so we can sleep. This means we can +use a mutex to protect the cache and all the objects within +it. Here's the code: + + + +#include <linux/list.h> +#include <linux/slab.h> +#include <linux/string.h> +#include <linux/mutex.h> +#include <asm/errno.h> + +struct object +{ + struct list_head list; + int id; + char name[32]; + int popularity; +}; + +/* Protects the cache, cache_num, and the objects within it */ +static DEFINE_MUTEX(cache_lock); +static LIST_HEAD(cache); +static unsigned int cache_num = 0; +#define MAX_CACHE_SIZE 10 + +/* Must be holding cache_lock */ +static struct object *__cache_find(int id) +{ + struct object *i; + + list_for_each_entry(i, &cache, list) + if (i->id == id) { + i->popularity++; + return i; + } + return NULL; +} + +/* Must be holding cache_lock */ +static void __cache_delete(struct object *obj) +{ + BUG_ON(!obj); + list_del(&obj->list); + kfree(obj); + cache_num--; +} + +/* Must be holding cache_lock */ +static void __cache_add(struct object *obj) +{ + list_add(&obj->list, &cache); + if (++cache_num > MAX_CACHE_SIZE) { + struct object *i, *outcast = NULL; + list_for_each_entry(i, &cache, list) { + if (!outcast || i->popularity < outcast->popularity) + outcast = i; + } + __cache_delete(outcast); + } +} + +int cache_add(int id, const char *name) +{ + struct object *obj; + + if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) + return -ENOMEM; + + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; + + mutex_lock(&cache_lock); + __cache_add(obj); + mutex_unlock(&cache_lock); + return 0; +} + +void cache_delete(int id) +{ + mutex_lock(&cache_lock); + __cache_delete(__cache_find(id)); + mutex_unlock(&cache_lock); +} + +int cache_find(int id, char *name) +{ + struct object *obj; + int ret = -ENOENT; + + mutex_lock(&cache_lock); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } + mutex_unlock(&cache_lock); + return ret; +} + + + +Note that we always make sure we have the cache_lock when we add, +delete, or look up the cache: both the cache infrastructure itself and +the contents of the objects are protected by the lock. In this case +it's easy, since we copy the data for the user, and never let them +access the objects directly. + + +There is a slight (and common) optimization here: in +cache_add we set up the fields of the object +before grabbing the lock. This is safe, as no-one else can access it +until we put it in cache. + + + + + Accessing From Interrupt Context + +Now consider the case where cache_find can be +called from interrupt context: either a hardware interrupt or a +softirq. An example would be a timer which deletes object from the +cache. + + +The change is shown below, in standard patch format: the +- are lines which are taken away, and the ++ are lines which are added. + + +--- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100 ++++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100 +@@ -12,7 +12,7 @@ + int popularity; + }; + +-static DEFINE_MUTEX(cache_lock); ++static DEFINE_SPINLOCK(cache_lock); + static LIST_HEAD(cache); + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 +@@ -55,6 +55,7 @@ + int cache_add(int id, const char *name) + { + struct object *obj; ++ unsigned long flags; + + if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) + return -ENOMEM; +@@ -63,30 +64,33 @@ + obj->id = id; + obj->popularity = 0; + +- mutex_lock(&cache_lock); ++ spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +- mutex_unlock(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + return 0; + } + + void cache_delete(int id) + { +- mutex_lock(&cache_lock); ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); + __cache_delete(__cache_find(id)); +- mutex_unlock(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + } + + int cache_find(int id, char *name) + { + struct object *obj; + int ret = -ENOENT; ++ unsigned long flags; + +- mutex_lock(&cache_lock); ++ spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } +- mutex_unlock(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + return ret; + } + + + +Note that the spin_lock_irqsave will turn off +interrupts if they are on, otherwise does nothing (if we are already +in an interrupt handler), hence these functions are safe to call from +any context. + + +Unfortunately, cache_add calls +kmalloc with the GFP_KERNEL +flag, which is only legal in user context. I have assumed that +cache_add is still only called in user context, +otherwise this should become a parameter to +cache_add. + + + + Exposing Objects Outside This File + +If our objects contained more information, it might not be sufficient +to copy the information in and out: other parts of the code might want +to keep pointers to these objects, for example, rather than looking up +the id every time. This produces two problems. + + +The first problem is that we use the cache_lock to +protect objects: we'd need to make this non-static so the rest of the +code can use it. This makes locking trickier, as it is no longer all +in one place. + + +The second problem is the lifetime problem: if another structure keeps +a pointer to an object, it presumably expects that pointer to remain +valid. Unfortunately, this is only guaranteed while you hold the +lock, otherwise someone might call cache_delete +and even worse, add another object, re-using the same address. + + +As there is only one lock, you can't hold it forever: no-one else would +get any work done. + + +The solution to this problem is to use a reference count: everyone who +has a pointer to the object increases it when they first get the +object, and drops the reference count when they're finished with it. +Whoever drops it to zero knows it is unused, and can actually delete it. + + +Here is the code: + + + +--- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100 ++++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100 +@@ -7,6 +7,7 @@ + struct object + { + struct list_head list; ++ unsigned int refcnt; + int id; + char name[32]; + int popularity; +@@ -17,6 +18,35 @@ + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 + ++static void __object_put(struct object *obj) ++{ ++ if (--obj->refcnt == 0) ++ kfree(obj); ++} ++ ++static void __object_get(struct object *obj) ++{ ++ obj->refcnt++; ++} ++ ++void object_put(struct object *obj) ++{ ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); ++ __object_put(obj); ++ spin_unlock_irqrestore(&cache_lock, flags); ++} ++ ++void object_get(struct object *obj) ++{ ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); ++ __object_get(obj); ++ spin_unlock_irqrestore(&cache_lock, flags); ++} ++ + /* Must be holding cache_lock */ + static struct object *__cache_find(int id) + { +@@ -35,6 +65,7 @@ + { + BUG_ON(!obj); + list_del(&obj->list); ++ __object_put(obj); + cache_num--; + } + +@@ -63,6 +94,7 @@ + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; ++ obj->refcnt = 1; /* The cache holds a reference */ + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +@@ -79,18 +111,15 @@ + spin_unlock_irqrestore(&cache_lock, flags); + } + +-int cache_find(int id, char *name) ++struct object *cache_find(int id) + { + struct object *obj; +- int ret = -ENOENT; + unsigned long flags; + + spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); +- if (obj) { +- ret = 0; +- strcpy(name, obj->name); +- } ++ if (obj) ++ __object_get(obj); + spin_unlock_irqrestore(&cache_lock, flags); +- return ret; ++ return obj; + } + + + +We encapsulate the reference counting in the standard 'get' and 'put' +functions. Now we can return the object itself from +cache_find which has the advantage that the user +can now sleep holding the object (eg. to +copy_to_user to name to userspace). + + +The other point to note is that I said a reference should be held for +every pointer to the object: thus the reference count is 1 when first +inserted into the cache. In some versions the framework does not hold +a reference count, but they are more complicated. + + + + Using Atomic Operations For The Reference Count + +In practice, atomic_t would usually be used for +refcnt. There are a number of atomic +operations defined in + +include/asm/atomic.h: these are +guaranteed to be seen atomically from all CPUs in the system, so no +lock is required. In this case, it is simpler than using spinlocks, +although for anything non-trivial using spinlocks is clearer. The +atomic_inc and +atomic_dec_and_test are used instead of the +standard increment and decrement operators, and the lock is no longer +used to protect the reference count itself. + + + +--- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100 ++++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100 +@@ -7,7 +7,7 @@ + struct object + { + struct list_head list; +- unsigned int refcnt; ++ atomic_t refcnt; + int id; + char name[32]; + int popularity; +@@ -18,33 +18,15 @@ + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 + +-static void __object_put(struct object *obj) +-{ +- if (--obj->refcnt == 0) +- kfree(obj); +-} +- +-static void __object_get(struct object *obj) +-{ +- obj->refcnt++; +-} +- + void object_put(struct object *obj) + { +- unsigned long flags; +- +- spin_lock_irqsave(&cache_lock, flags); +- __object_put(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ if (atomic_dec_and_test(&obj->refcnt)) ++ kfree(obj); + } + + void object_get(struct object *obj) + { +- unsigned long flags; +- +- spin_lock_irqsave(&cache_lock, flags); +- __object_get(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ atomic_inc(&obj->refcnt); + } + + /* Must be holding cache_lock */ +@@ -65,7 +47,7 @@ + { + BUG_ON(!obj); + list_del(&obj->list); +- __object_put(obj); ++ object_put(obj); + cache_num--; + } + +@@ -94,7 +76,7 @@ + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; +- obj->refcnt = 1; /* The cache holds a reference */ ++ atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +@@ -119,7 +101,7 @@ + spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); + if (obj) +- __object_get(obj); ++ object_get(obj); + spin_unlock_irqrestore(&cache_lock, flags); + return obj; + } + + + + + + Protecting The Objects Themselves + +In these examples, we assumed that the objects (except the reference +counts) never changed once they are created. If we wanted to allow +the name to change, there are three possibilities: + + + + +You can make cache_lock non-static, and tell people +to grab that lock before changing the name in any object. + + + + +You can provide a cache_obj_rename which grabs +this lock and changes the name for the caller, and tell everyone to +use that function. + + + + +You can make the cache_lock protect only the cache +itself, and use another lock to protect the name. + + + + + +Theoretically, you can make the locks as fine-grained as one lock for +every field, for every object. In practice, the most common variants +are: + + + + +One lock which protects the infrastructure (the cache +list in this example) and all the objects. This is what we have done +so far. + + + + +One lock which protects the infrastructure (including the list +pointers inside the objects), and one lock inside the object which +protects the rest of that object. + + + + +Multiple locks to protect the infrastructure (eg. one lock per hash +chain), possibly with a separate per-object lock. + + + + + +Here is the "lock-per-object" implementation: + + +--- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100 ++++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 +@@ -6,11 +6,17 @@ + + struct object + { ++ /* These two protected by cache_lock. */ + struct list_head list; ++ int popularity; ++ + atomic_t refcnt; ++ ++ /* Doesn't change once created. */ + int id; ++ ++ spinlock_t lock; /* Protects the name */ + char name[32]; +- int popularity; + }; + + static DEFINE_SPINLOCK(cache_lock); +@@ -77,6 +84,7 @@ + obj->id = id; + obj->popularity = 0; + atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ ++ spin_lock_init(&obj->lock); + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); + + + +Note that I decide that the popularity +count should be protected by the cache_lock rather +than the per-object lock: this is because it (like the +struct list_head inside the object) is +logically part of the infrastructure. This way, I don't need to grab +the lock of every object in __cache_add when +seeking the least popular. + + + +I also decided that the id member is +unchangeable, so I don't need to grab each object lock in +__cache_find() to examine the +id: the object lock is only used by a +caller who wants to read or write the name +field. + + + +Note also that I added a comment describing what data was protected by +which locks. This is extremely important, as it describes the runtime +behavior of the code, and can be hard to gain from just reading. And +as Alan Cox says, Lock data, not code. + + + + + + Common Problems + + Deadlock: Simple and Advanced + + + There is a coding bug where a piece of code tries to grab a + spinlock twice: it will spin forever, waiting for the lock to + be released (spinlocks, rwlocks and mutexes are not + recursive in Linux). This is trivial to diagnose: not a + stay-up-five-nights-talk-to-fluffy-code-bunnies kind of + problem. + + + + For a slightly more complex case, imagine you have a region + shared by a softirq and user context. If you use a + spin_lock() call to protect it, it is + possible that the user context will be interrupted by the softirq + while it holds the lock, and the softirq will then spin + forever trying to get the same lock. + + + + Both of these are called deadlock, and as shown above, it can + occur even with a single CPU (although not on UP compiles, + since spinlocks vanish on kernel compiles with + CONFIG_SMP=n. You'll still get data corruption + in the second example). + + + + This complete lockup is easy to diagnose: on SMP boxes the + watchdog timer or compiling with DEBUG_SPINLOCK set + (include/linux/spinlock.h) will show this up + immediately when it happens. + + + + A more complex problem is the so-called 'deadly embrace', + involving two or more locks. Say you have a hash table: each + entry in the table is a spinlock, and a chain of hashed + objects. Inside a softirq handler, you sometimes want to + alter an object from one place in the hash to another: you + grab the spinlock of the old hash chain and the spinlock of + the new hash chain, and delete the object from the old one, + and insert it in the new one. + + + + There are two problems here. First, if your code ever + tries to move the object to the same chain, it will deadlock + with itself as it tries to lock it twice. Secondly, if the + same softirq on another CPU is trying to move another object + in the reverse direction, the following could happen: + + + + Consequences + + + + + + CPU 1 + CPU 2 + + + + + + Grab lock A -> OK + Grab lock B -> OK + + + Grab lock B -> spin + Grab lock A -> spin + + + +
+ + + The two CPUs will spin forever, waiting for the other to give up + their lock. It will look, smell, and feel like a crash. + + + + + Preventing Deadlock + + + Textbooks will tell you that if you always lock in the same + order, you will never get this kind of deadlock. Practice + will tell you that this approach doesn't scale: when I + create a new lock, I don't understand enough of the kernel + to figure out where in the 5000 lock hierarchy it will fit. + + + + The best locks are encapsulated: they never get exposed in + headers, and are never held around calls to non-trivial + functions outside the same file. You can read through this + code and see that it will never deadlock, because it never + tries to grab another lock while it has that one. People + using your code don't even need to know you are using a + lock. + + + + A classic problem here is when you provide callbacks or + hooks: if you call these with the lock held, you risk simple + deadlock, or a deadly embrace (who knows what the callback + will do?). Remember, the other programmers are out to get + you, so don't do this. + + + + Overzealous Prevention Of Deadlocks + + + Deadlocks are problematic, but not as bad as data + corruption. Code which grabs a read lock, searches a list, + fails to find what it wants, drops the read lock, grabs a + write lock and inserts the object has a race condition. + + + + If you don't see why, please stay the fuck away from my code. + + + + + + Racing Timers: A Kernel Pastime + + + Timers can produce their own special problems with races. + Consider a collection of objects (list, hash, etc) where each + object has a timer which is due to destroy it. + + + + If you want to destroy the entire collection (say on module + removal), you might do the following: + + + + /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE + HUNGARIAN NOTATION */ + spin_lock_bh(&list_lock); + + while (list) { + struct foo *next = list->next; + del_timer(&list->timer); + kfree(list); + list = next; + } + + spin_unlock_bh(&list_lock); + + + + Sooner or later, this will crash on SMP, because a timer can + have just gone off before the spin_lock_bh(), + and it will only get the lock after we + spin_unlock_bh(), and then try to free + the element (which has already been freed!). + + + + This can be avoided by checking the result of + del_timer(): if it returns + 1, the timer has been deleted. + If 0, it means (in this + case) that it is currently running, so we can do: + + + + retry: + spin_lock_bh(&list_lock); + + while (list) { + struct foo *next = list->next; + if (!del_timer(&list->timer)) { + /* Give timer a chance to delete this */ + spin_unlock_bh(&list_lock); + goto retry; + } + kfree(list); + list = next; + } + + spin_unlock_bh(&list_lock); + + + + Another common problem is deleting timers which restart + themselves (by calling add_timer() at the end + of their timer function). Because this is a fairly common case + which is prone to races, you should use del_timer_sync() + (include/linux/timer.h) + to handle this case. It returns the number of times the timer + had to be deleted before we finally stopped it from adding itself back + in. + + + + + + + Locking Speed + + +There are three main things to worry about when considering speed of +some code which does locking. First is concurrency: how many things +are going to be waiting while someone else is holding a lock. Second +is the time taken to actually acquire and release an uncontended lock. +Third is using fewer, or smarter locks. I'm assuming that the lock is +used fairly often: otherwise, you wouldn't be concerned about +efficiency. + + +Concurrency depends on how long the lock is usually held: you should +hold the lock for as long as needed, but no longer. In the cache +example, we always create the object without the lock held, and then +grab the lock only when we are ready to insert it in the list. + + +Acquisition times depend on how much damage the lock operations do to +the pipeline (pipeline stalls) and how likely it is that this CPU was +the last one to grab the lock (ie. is the lock cache-hot for this +CPU): on a machine with more CPUs, this likelihood drops fast. +Consider a 700MHz Intel Pentium III: an instruction takes about 0.7ns, +an atomic increment takes about 58ns, a lock which is cache-hot on +this CPU takes 160ns, and a cacheline transfer from another CPU takes +an additional 170 to 360ns. (These figures from Paul McKenney's + Linux +Journal RCU article). + + +These two aims conflict: holding a lock for a short time might be done +by splitting locks into parts (such as in our final per-object-lock +example), but this increases the number of lock acquisitions, and the +results are often slower than having a single lock. This is another +reason to advocate locking simplicity. + + +The third concern is addressed below: there are some methods to reduce +the amount of locking which needs to be done. + + + + Read/Write Lock Variants + + + Both spinlocks and mutexes have read/write variants: + rwlock_t and struct rw_semaphore. + These divide users into two classes: the readers and the writers. If + you are only reading the data, you can get a read lock, but to write to + the data you need the write lock. Many people can hold a read lock, + but a writer must be sole holder. + + + + If your code divides neatly along reader/writer lines (as our + cache code does), and the lock is held by readers for + significant lengths of time, using these locks can help. They + are slightly slower than the normal locks though, so in practice + rwlock_t is not usually worthwhile. + + + + + Avoiding Locks: Read Copy Update + + + There is a special method of read/write locking called Read Copy + Update. Using RCU, the readers can avoid taking a lock + altogether: as we expect our cache to be read more often than + updated (otherwise the cache is a waste of time), it is a + candidate for this optimization. + + + + How do we get rid of read locks? Getting rid of read locks + means that writers may be changing the list underneath the + readers. That is actually quite simple: we can read a linked + list while an element is being added if the writer adds the + element very carefully. For example, adding + new to a single linked list called + list: + + + + new->next = list->next; + wmb(); + list->next = new; + + + + The wmb() is a write memory barrier. It + ensures that the first operation (setting the new element's + next pointer) is complete and will be seen by + all CPUs, before the second operation is (putting the new + element into the list). This is important, since modern + compilers and modern CPUs can both reorder instructions unless + told otherwise: we want a reader to either not see the new + element at all, or see the new element with the + next pointer correctly pointing at the rest of + the list. + + + Fortunately, there is a function to do this for standard + struct list_head lists: + list_add_rcu() + (include/linux/list.h). + + + Removing an element from the list is even simpler: we replace + the pointer to the old element with a pointer to its successor, + and readers will either see it, or skip over it. + + + list->next = old->next; + + + There is list_del_rcu() + (include/linux/list.h) which does this (the + normal version poisons the old object, which we don't want). + + + The reader must also be careful: some CPUs can look through the + next pointer to start reading the contents of + the next element early, but don't realize that the pre-fetched + contents is wrong when the next pointer changes + underneath them. Once again, there is a + list_for_each_entry_rcu() + (include/linux/list.h) to help you. Of + course, writers can just use + list_for_each_entry(), since there cannot + be two simultaneous writers. + + + Our final dilemma is this: when can we actually destroy the + removed element? Remember, a reader might be stepping through + this element in the list right now: if we free this element and + the next pointer changes, the reader will jump + off into garbage and crash. We need to wait until we know that + all the readers who were traversing the list when we deleted the + element are finished. We use call_rcu() to + register a callback which will actually destroy the object once + all pre-existing readers are finished. Alternatively, + synchronize_rcu() may be used to block until + all pre-existing are finished. + + + But how does Read Copy Update know when the readers are + finished? The method is this: firstly, the readers always + traverse the list inside + rcu_read_lock()/rcu_read_unlock() + pairs: these simply disable preemption so the reader won't go to + sleep while reading the list. + + + RCU then waits until every other CPU has slept at least once: + since readers cannot sleep, we know that any readers which were + traversing the list during the deletion are finished, and the + callback is triggered. The real Read Copy Update code is a + little more optimized than this, but this is the fundamental + idea. + + + +--- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 ++++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100 +@@ -1,15 +1,18 @@ + #include <linux/list.h> + #include <linux/slab.h> + #include <linux/string.h> ++#include <linux/rcupdate.h> + #include <linux/mutex.h> + #include <asm/errno.h> + + struct object + { +- /* These two protected by cache_lock. */ ++ /* This is protected by RCU */ + struct list_head list; + int popularity; + ++ struct rcu_head rcu; ++ + atomic_t refcnt; + + /* Doesn't change once created. */ +@@ -40,7 +43,7 @@ + { + struct object *i; + +- list_for_each_entry(i, &cache, list) { ++ list_for_each_entry_rcu(i, &cache, list) { + if (i->id == id) { + i->popularity++; + return i; +@@ -49,19 +52,25 @@ + return NULL; + } + ++/* Final discard done once we know no readers are looking. */ ++static void cache_delete_rcu(void *arg) ++{ ++ object_put(arg); ++} ++ + /* Must be holding cache_lock */ + static void __cache_delete(struct object *obj) + { + BUG_ON(!obj); +- list_del(&obj->list); +- object_put(obj); ++ list_del_rcu(&obj->list); + cache_num--; ++ call_rcu(&obj->rcu, cache_delete_rcu); + } + + /* Must be holding cache_lock */ + static void __cache_add(struct object *obj) + { +- list_add(&obj->list, &cache); ++ list_add_rcu(&obj->list, &cache); + if (++cache_num > MAX_CACHE_SIZE) { + struct object *i, *outcast = NULL; + list_for_each_entry(i, &cache, list) { +@@ -104,12 +114,11 @@ + struct object *cache_find(int id) + { + struct object *obj; +- unsigned long flags; + +- spin_lock_irqsave(&cache_lock, flags); ++ rcu_read_lock(); + obj = __cache_find(id); + if (obj) + object_get(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ rcu_read_unlock(); + return obj; + } + + + +Note that the reader will alter the +popularity member in +__cache_find(), and now it doesn't hold a lock. +One solution would be to make it an atomic_t, but for +this usage, we don't really care about races: an approximate result is +good enough, so I didn't change it. + + + +The result is that cache_find() requires no +synchronization with any other functions, so is almost as fast on SMP +as it would be on UP. + + + +There is a furthur optimization possible here: remember our original +cache code, where there were no reference counts and the caller simply +held the lock whenever using the object? This is still possible: if +you hold the lock, no one can delete the object, so you don't need to +get and put the reference count. + + + +Now, because the 'read lock' in RCU is simply disabling preemption, a +caller which always has preemption disabled between calling +cache_find() and +object_put() does not need to actually get and +put the reference count: we could expose +__cache_find() by making it non-static, and +such callers could simply call that. + + +The benefit here is that the reference count is not written to: the +object is not altered in any way, which is much faster on SMP +machines due to caching. + + + + + Per-CPU Data + + + Another technique for avoiding locking which is used fairly + widely is to duplicate information for each CPU. For example, + if you wanted to keep a count of a common condition, you could + use a spin lock and a single counter. Nice and simple. + + + + If that was too slow (it's usually not, but if you've got a + really big machine to test on and can show that it is), you + could instead use a counter for each CPU, then none of them need + an exclusive lock. See DEFINE_PER_CPU(), + get_cpu_var() and + put_cpu_var() + (include/linux/percpu.h). + + + + Of particular use for simple per-cpu counters is the + local_t type, and the + cpu_local_inc() and related functions, + which are more efficient than simple code on some architectures + (include/asm/local.h). + + + + Note that there is no simple, reliable way of getting an exact + value of such a counter, without introducing more locks. This + is not a problem for some uses. + + + + + Data Which Mostly Used By An IRQ Handler + + + If data is always accessed from within the same IRQ handler, you + don't need a lock at all: the kernel already guarantees that the + irq handler will not run simultaneously on multiple CPUs. + + + Manfred Spraul points out that you can still do this, even if + the data is very occasionally accessed in user context or + softirqs/tasklets. The irq handler doesn't use a lock, and + all other accesses are done as so: + + + + spin_lock(&lock); + disable_irq(irq); + ... + enable_irq(irq); + spin_unlock(&lock); + + + The disable_irq() prevents the irq handler + from running (and waits for it to finish if it's currently + running on other CPUs). The spinlock prevents any other + accesses happening at the same time. Naturally, this is slower + than just a spin_lock_irq() call, so it + only makes sense if this type of access happens extremely + rarely. + + + + + + What Functions Are Safe To Call From Interrupts? + + + Many functions in the kernel sleep (ie. call schedule()) + directly or indirectly: you can never call them while holding a + spinlock, or with preemption disabled. This also means you need + to be in user context: calling them from an interrupt is illegal. + + + + Some Functions Which Sleep + + + The most common ones are listed below, but you usually have to + read the code to find out if other calls are safe. If everyone + else who calls it can sleep, you probably need to be able to + sleep, too. In particular, registration and deregistration + functions usually expect to be called from user context, and can + sleep. + + + + + + Accesses to + userspace: + + + + + copy_from_user() + + + + + copy_to_user() + + + + + get_user() + + + + + put_user() + + + + + + + + kmalloc(GFP_KERNEL) + + + + + + mutex_lock_interruptible() and + mutex_lock() + + + There is a mutex_trylock() which does not + sleep. Still, it must not be used inside interrupt context since + its implementation is not safe for that. + mutex_unlock() will also never sleep. + It cannot be used in interrupt context either since a mutex + must be released by the same task that acquired it. + + + + + + + Some Functions Which Don't Sleep + + + Some functions are safe to call from any context, or holding + almost any lock. + + + + + + printk() + + + + + kfree() + + + + + add_timer() and del_timer() + + + + + + + + Mutex API reference +!Iinclude/linux/mutex.h +!Ekernel/mutex.c + + + + Further reading + + + + + Documentation/spinlocks.txt: + Linus Torvalds' spinlocking tutorial in the kernel sources. + + + + + + Unix Systems for Modern Architectures: Symmetric + Multiprocessing and Caching for Kernel Programmers: + + + + Curt Schimmel's very good introduction to kernel level + locking (not written for Linux, but nearly everything + applies). The book is expensive, but really worth every + penny to understand SMP locking. [ISBN: 0201633388] + + + + + + + Thanks + + + Thanks to Telsa Gwynne for DocBooking, neatening and adding + style. + + + + Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul + Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim + Waugh, Pete Zaitcev, James Morris, Robert Love, Paul McKenney, + John Ashby for proofreading, correcting, flaming, commenting. + + + + Thanks to the cabal for having no influence on this document. + + + + + Glossary + + + preemption + + + Prior to 2.5, or when CONFIG_PREEMPT is + unset, processes in user context inside the kernel would not + preempt each other (ie. you had that CPU until you gave it up, + except for interrupts). With the addition of + CONFIG_PREEMPT in 2.5.4, this changed: when + in user context, higher priority tasks can "cut in": spinlocks + were changed to disable preemption, even on UP. + + + + + + bh + + + Bottom Half: for historical reasons, functions with + '_bh' in them often now refer to any software interrupt, e.g. + spin_lock_bh() blocks any software interrupt + on the current CPU. Bottom halves are deprecated, and will + eventually be replaced by tasklets. Only one bottom half will be + running at any time. + + + + + + Hardware Interrupt / Hardware IRQ + + + Hardware interrupt request. in_irq() returns + true in a hardware interrupt handler. + + + + + + Interrupt Context + + + Not user context: processing a hardware irq or software irq. + Indicated by the in_interrupt() macro + returning true. + + + + + + SMP + + + Symmetric Multi-Processor: kernels compiled for multiple-CPU + machines. (CONFIG_SMP=y). + + + + + + Software Interrupt / softirq + + + Software interrupt handler. in_irq() returns + false; in_softirq() + returns true. Tasklets and softirqs + both fall into the category of 'software interrupts'. + + + Strictly speaking a softirq is one of up to 32 enumerated software + interrupts which can run on multiple CPUs at once. + Sometimes used to refer to tasklets as + well (ie. all software interrupts). + + + + + + tasklet + + + A dynamically-registrable software interrupt, + which is guaranteed to only run on one CPU at a time. + + + + + + timer + + + A dynamically-registrable software interrupt, which is run at + (or close to) a given time. When running, it is just like a + tasklet (in fact, they are called from the TIMER_SOFTIRQ). + + + + + + UP + + + Uni-Processor: Non-SMP. (CONFIG_SMP=n). + + + + + + User Context + + + The kernel executing on behalf of a particular process (ie. a + system call or trap) or kernel thread. You can tell which + process with the current macro.) Not to + be confused with userspace. Can be interrupted by software or + hardware interrupts. + + + + + + Userspace + + + A process executing its own code outside the kernel. + + + + + + + -- cgit v1.2.3