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-hacking.tmpl | 1327 +++++++++++++++++++++++++++++ 1 file changed, 1327 insertions(+) create mode 100644 Documentation/DocBook/kernel-hacking.tmpl (limited to 'Documentation/DocBook/kernel-hacking.tmpl') diff --git a/Documentation/DocBook/kernel-hacking.tmpl b/Documentation/DocBook/kernel-hacking.tmpl new file mode 100644 index 00000000..7b3f4936 --- /dev/null +++ b/Documentation/DocBook/kernel-hacking.tmpl @@ -0,0 +1,1327 @@ + + + + + + Unreliable Guide To Hacking The Linux Kernel + + + + Rusty + Russell + +
+ rusty@rustcorp.com.au +
+ + + + + + 2005 + 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. + + + + + This is the first release of this document as part of the kernel tarball. + + + + + + + + Introduction + + Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux + Kernel Hacking. This document describes the common routines and + general requirements for kernel code: its goal is to serve as a + primer for Linux kernel development for experienced C + programmers. I avoid implementation details: that's what the + code is for, and I ignore whole tracts of useful routines. + + + Before you read this, please understand that I never wanted to + write this document, being grossly under-qualified, but I always + wanted to read it, and this was the only way. I hope it will + grow into a compendium of best practice, common starting points + and random information. + + + + + The Players + + + At any time each of the CPUs in a system can be: + + + + + + not associated with any process, serving a hardware interrupt; + + + + + + not associated with any process, serving a softirq or tasklet; + + + + + + running in kernel space, associated with a process (user context); + + + + + + running a process in user space. + + + + + + There is an ordering between these. The bottom two can preempt + each other, but above that is a strict hierarchy: each can only be + preempted by the ones above it. For example, while a softirq is + running on a CPU, no other softirq will preempt it, but a hardware + interrupt can. However, any other CPUs in the system execute + independently. + + + + We'll see a number of ways that the user context can block + interrupts, to become truly non-preemptable. + + + + User Context + + + User context is when you are coming in from a system call or other + trap: like userspace, you can be preempted by more important tasks + and by interrupts. You can sleep, by calling + schedule(). + + + + + You are always in user context on module load and unload, + and on operations on the block device layer. + + + + + In user context, the current pointer (indicating + the task we are currently executing) is valid, and + in_interrupt() + (include/linux/interrupt.h) is false + . + + + + + Beware that if you have preemption or softirqs disabled + (see below), in_interrupt() will return a + false positive. + + + + + + Hardware Interrupts (Hard IRQs) + + + Timer ticks, network cards and + keyboard are examples of real + hardware which produce interrupts at any time. The kernel runs + interrupt handlers, which services the hardware. The kernel + guarantees that this handler is never re-entered: if the same + interrupt arrives, it is queued (or dropped). Because it + disables interrupts, this handler has to be fast: frequently it + simply acknowledges the interrupt, marks a 'software interrupt' + for execution and exits. + + + + You can tell you are in a hardware interrupt, because + in_irq() returns true. + + + + Beware that this will return a false positive if interrupts are disabled + (see below). + + + + + + Software Interrupt Context: Softirqs and Tasklets + + + Whenever a system call is about to return to userspace, or a + hardware interrupt handler exits, any 'software interrupts' + which are marked pending (usually by hardware interrupts) are + run (kernel/softirq.c). + + + + Much of the real interrupt handling work is done here. Early in + the transition to SMP, there were only 'bottom + halves' (BHs), which didn't take advantage of multiple CPUs. Shortly + after we switched from wind-up computers made of match-sticks and snot, + we abandoned this limitation and switched to 'softirqs'. + + + + include/linux/interrupt.h lists the + different softirqs. A very important softirq is the + timer softirq (include/linux/timer.h): you can + register to have it call functions for you in a given length of + time. + + + + Softirqs are often a pain to deal with, since the same softirq + will run simultaneously on more than one CPU. For this reason, + tasklets (include/linux/interrupt.h) are more + often used: they are dynamically-registrable (meaning you can have + as many as you want), and they also guarantee that any tasklet + will only run on one CPU at any time, although different tasklets + can run simultaneously. + + + + The name 'tasklet' is misleading: they have nothing to do with 'tasks', + and probably more to do with some bad vodka Alexey Kuznetsov had at the + time. + + + + + You can tell you are in a softirq (or tasklet) + using the in_softirq() macro + (include/linux/interrupt.h). + + + + Beware that this will return a false positive if a bh lock (see below) + is held. + + + + + + + Some Basic Rules + + + + No memory protection + + + If you corrupt memory, whether in user context or + interrupt context, the whole machine will crash. Are you + sure you can't do what you want in userspace? + + + + + + No floating point or MMX + + + The FPU context is not saved; even in user + context the FPU state probably won't + correspond with the current process: you would mess with some + user process' FPU state. If you really want + to do this, you would have to explicitly save/restore the full + FPU state (and avoid context switches). It + is generally a bad idea; use fixed point arithmetic first. + + + + + + A rigid stack limit + + + Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's + about 14K on most 64-bit archs, and often shared with interrupts + so you can't use it all. Avoid deep recursion and huge local + arrays on the stack (allocate them dynamically instead). + + + + + + The Linux kernel is portable + + + Let's keep it that way. Your code should be 64-bit clean, + and endian-independent. You should also minimize CPU + specific stuff, e.g. inline assembly should be cleanly + encapsulated and minimized to ease porting. Generally it + should be restricted to the architecture-dependent part of + the kernel tree. + + + + + + + + ioctls: Not writing a new system call + + + A system call generally looks like this + + + +asmlinkage long sys_mycall(int arg) +{ + return 0; +} + + + + First, in most cases you don't want to create a new system call. + You create a character device and implement an appropriate ioctl + for it. This is much more flexible than system calls, doesn't have + to be entered in every architecture's + include/asm/unistd.h and + arch/kernel/entry.S file, and is much more + likely to be accepted by Linus. + + + + If all your routine does is read or write some parameter, consider + implementing a sysfs interface instead. + + + + Inside the ioctl you're in user context to a process. When a + error occurs you return a negated errno (see + include/linux/errno.h), + otherwise you return 0. + + + + After you slept you should check if a signal occurred: the + Unix/Linux way of handling signals is to temporarily exit the + system call with the -ERESTARTSYS error. The + system call entry code will switch back to user context, process + the signal handler and then your system call will be restarted + (unless the user disabled that). So you should be prepared to + process the restart, e.g. if you're in the middle of manipulating + some data structure. + + + +if (signal_pending(current)) + return -ERESTARTSYS; + + + + If you're doing longer computations: first think userspace. If you + really want to do it in kernel you should + regularly check if you need to give up the CPU (remember there is + cooperative multitasking per CPU). Idiom: + + + +cond_resched(); /* Will sleep */ + + + + A short note on interface design: the UNIX system call motto is + "Provide mechanism not policy". + + + + + Recipes for Deadlock + + + You cannot call any routines which may sleep, unless: + + + + + You are in user context. + + + + + + You do not own any spinlocks. + + + + + + You have interrupts enabled (actually, Andi Kleen says + that the scheduling code will enable them for you, but + that's probably not what you wanted). + + + + + + Note that some functions may sleep implicitly: common ones are + the user space access functions (*_user) and memory allocation + functions without GFP_ATOMIC. + + + + You should always compile your kernel + CONFIG_DEBUG_SPINLOCK_SLEEP on, and it will warn + you if you break these rules. If you do break + the rules, you will eventually lock up your box. + + + + Really. + + + + + Common Routines + + + + <function>printk()</function> + <filename class="headerfile">include/linux/kernel.h</filename> + + + + printk() feeds kernel messages to the + console, dmesg, and the syslog daemon. It is useful for debugging + and reporting errors, and can be used inside interrupt context, + but use with caution: a machine which has its console flooded with + printk messages is unusable. It uses a format string mostly + compatible with ANSI C printf, and C string concatenation to give + it a first "priority" argument: + + + +printk(KERN_INFO "i = %u\n", i); + + + + See include/linux/kernel.h; + for other KERN_ values; these are interpreted by syslog as the + level. Special case: for printing an IP address use + + + +__be32 ipaddress; +printk(KERN_INFO "my ip: %pI4\n", &ipaddress); + + + + printk() internally uses a 1K buffer and does + not catch overruns. Make sure that will be enough. + + + + + You will know when you are a real kernel hacker + when you start typoing printf as printk in your user programs :) + + + + + + + + Another sidenote: the original Unix Version 6 sources had a + comment on top of its printf function: "Printf should not be + used for chit-chat". You should follow that advice. + + + + + + + <function>copy_[to/from]_user()</function> + / + <function>get_user()</function> + / + <function>put_user()</function> + <filename class="headerfile">include/asm/uaccess.h</filename> + + + + [SLEEPS] + + + + put_user() and get_user() + are used to get and put single values (such as an int, char, or + long) from and to userspace. A pointer into userspace should + never be simply dereferenced: data should be copied using these + routines. Both return -EFAULT or 0. + + + copy_to_user() and + copy_from_user() are more general: they copy + an arbitrary amount of data to and from userspace. + + + Unlike put_user() and + get_user(), they return the amount of + uncopied data (ie. 0 still means + success). + + + [Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.] + + + The functions may sleep implicitly. This should never be called + outside user context (it makes no sense), with interrupts + disabled, or a spinlock held. + + + + + <function>kmalloc()</function>/<function>kfree()</function> + <filename class="headerfile">include/linux/slab.h</filename> + + + [MAY SLEEP: SEE BELOW] + + + + These routines are used to dynamically request pointer-aligned + chunks of memory, like malloc and free do in userspace, but + kmalloc() takes an extra flag word. + Important values: + + + + + + + GFP_KERNEL + + + + + May sleep and swap to free memory. Only allowed in user + context, but is the most reliable way to allocate memory. + + + + + + + + GFP_ATOMIC + + + + + Don't sleep. Less reliable than GFP_KERNEL, + but may be called from interrupt context. You should + really have a good out-of-memory + error-handling strategy. + + + + + + + + GFP_DMA + + + + + Allocate ISA DMA lower than 16MB. If you don't know what that + is you don't need it. Very unreliable. + + + + + + + If you see a sleeping function called from invalid + context warning message, then maybe you called a + sleeping allocation function from interrupt context without + GFP_ATOMIC. You should really fix that. + Run, don't walk. + + + + If you are allocating at least PAGE_SIZE + (include/asm/page.h) bytes, + consider using __get_free_pages() + + (include/linux/mm.h). It + takes an order argument (0 for page sized, 1 for double page, 2 + for four pages etc.) and the same memory priority flag word as + above. + + + + If you are allocating more than a page worth of bytes you can use + vmalloc(). It'll allocate virtual memory in + the kernel map. This block is not contiguous in physical memory, + but the MMU makes it look like it is for you + (so it'll only look contiguous to the CPUs, not to external device + drivers). If you really need large physically contiguous memory + for some weird device, you have a problem: it is poorly supported + in Linux because after some time memory fragmentation in a running + kernel makes it hard. The best way is to allocate the block early + in the boot process via the alloc_bootmem() + routine. + + + + Before inventing your own cache of often-used objects consider + using a slab cache in + include/linux/slab.h + + + + + <function>current</function> + <filename class="headerfile">include/asm/current.h</filename> + + + This global variable (really a macro) contains a pointer to + the current task structure, so is only valid in user context. + For example, when a process makes a system call, this will + point to the task structure of the calling process. It is + not NULL in interrupt context. + + + + + <function>mdelay()</function>/<function>udelay()</function> + <filename class="headerfile">include/asm/delay.h</filename> + <filename class="headerfile">include/linux/delay.h</filename> + + + + The udelay() and ndelay() functions can be used for small pauses. + Do not use large values with them as you risk + overflow - the helper function mdelay() is useful + here, or consider msleep(). + + + + + <function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function> + <filename class="headerfile">include/asm/byteorder.h</filename> + + + + The cpu_to_be32() family (where the "32" can + be replaced by 64 or 16, and the "be" can be replaced by "le") are + the general way to do endian conversions in the kernel: they + return the converted value. All variations supply the reverse as + well: be32_to_cpu(), etc. + + + + There are two major variations of these functions: the pointer + variation, such as cpu_to_be32p(), which take + a pointer to the given type, and return the converted value. The + other variation is the "in-situ" family, such as + cpu_to_be32s(), which convert value referred + to by the pointer, and return void. + + + + + <function>local_irq_save()</function>/<function>local_irq_restore()</function> + <filename class="headerfile">include/asm/system.h</filename> + + + + These routines disable hard interrupts on the local CPU, and + restore them. They are reentrant; saving the previous state in + their one unsigned long flags argument. If you + know that interrupts are enabled, you can simply use + local_irq_disable() and + local_irq_enable(). + + + + + <function>local_bh_disable()</function>/<function>local_bh_enable()</function> + <filename class="headerfile">include/linux/interrupt.h</filename> + + + These routines disable soft interrupts on the local CPU, and + restore them. They are reentrant; if soft interrupts were + disabled before, they will still be disabled after this pair + of functions has been called. They prevent softirqs and tasklets + from running on the current CPU. + + + + + <function>smp_processor_id</function>() + <filename class="headerfile">include/asm/smp.h</filename> + + + get_cpu() disables preemption (so you won't + suddenly get moved to another CPU) and returns the current + processor number, between 0 and NR_CPUS. Note + that the CPU numbers are not necessarily continuous. You return + it again with put_cpu() when you are done. + + + If you know you cannot be preempted by another task (ie. you are + in interrupt context, or have preemption disabled) you can use + smp_processor_id(). + + + + + <type>__init</type>/<type>__exit</type>/<type>__initdata</type> + <filename class="headerfile">include/linux/init.h</filename> + + + After boot, the kernel frees up a special section; functions + marked with __init and data structures marked with + __initdata are dropped after boot is complete: similarly + modules discard this memory after initialization. __exit + is used to declare a function which is only required on exit: the + function will be dropped if this file is not compiled as a module. + See the header file for use. Note that it makes no sense for a function + marked with __init to be exported to modules with + EXPORT_SYMBOL() - this will break. + + + + + + <function>__initcall()</function>/<function>module_init()</function> + <filename class="headerfile">include/linux/init.h</filename> + + Many parts of the kernel are well served as a module + (dynamically-loadable parts of the kernel). Using the + module_init() and + module_exit() macros it is easy to write code + without #ifdefs which can operate both as a module or built into + the kernel. + + + + The module_init() macro defines which + function is to be called at module insertion time (if the file is + compiled as a module), or at boot time: if the file is not + compiled as a module the module_init() macro + becomes equivalent to __initcall(), which + through linker magic ensures that the function is called on boot. + + + + The function can return a negative error number to cause + module loading to fail (unfortunately, this has no effect if + the module is compiled into the kernel). This function is + called in user context with interrupts enabled, so it can sleep. + + + + + <function>module_exit()</function> + <filename class="headerfile">include/linux/init.h</filename> + + + This macro defines the function to be called at module removal + time (or never, in the case of the file compiled into the + kernel). It will only be called if the module usage count has + reached zero. This function can also sleep, but cannot fail: + everything must be cleaned up by the time it returns. + + + + Note that this macro is optional: if it is not present, your + module will not be removable (except for 'rmmod -f'). + + + + + <function>try_module_get()</function>/<function>module_put()</function> + <filename class="headerfile">include/linux/module.h</filename> + + + These manipulate the module usage count, to protect against + removal (a module also can't be removed if another module uses one + of its exported symbols: see below). Before calling into module + code, you should call try_module_get() on + that module: if it fails, then the module is being removed and you + should act as if it wasn't there. Otherwise, you can safely enter + the module, and call module_put() when you're + finished. + + + + Most registerable structures have an + owner field, such as in the + file_operations structure. Set this field + to the macro THIS_MODULE. + + + + + + + + Wait Queues + <filename class="headerfile">include/linux/wait.h</filename> + + + [SLEEPS] + + + + A wait queue is used to wait for someone to wake you up when a + certain condition is true. They must be used carefully to ensure + there is no race condition. You declare a + wait_queue_head_t, and then processes which want to + wait for that condition declare a wait_queue_t + referring to themselves, and place that in the queue. + + + + Declaring + + + You declare a wait_queue_head_t using the + DECLARE_WAIT_QUEUE_HEAD() macro, or using the + init_waitqueue_head() routine in your + initialization code. + + + + + Queuing + + + Placing yourself in the waitqueue is fairly complex, because you + must put yourself in the queue before checking the condition. + There is a macro to do this: + wait_event_interruptible() + + include/linux/wait.h The + first argument is the wait queue head, and the second is an + expression which is evaluated; the macro returns + 0 when this expression is true, or + -ERESTARTSYS if a signal is received. + The wait_event() version ignores signals. + + + Do not use the sleep_on() function family - + it is very easy to accidentally introduce races; almost certainly + one of the wait_event() family will do, or a + loop around schedule_timeout(). If you choose + to loop around schedule_timeout() remember + you must set the task state (with + set_current_state()) on each iteration to avoid + busy-looping. + + + + + + Waking Up Queued Tasks + + + Call wake_up() + + include/linux/wait.h;, + which will wake up every process in the queue. The exception is + if one has TASK_EXCLUSIVE set, in which case + the remainder of the queue will not be woken. There are other variants + of this basic function available in the same header. + + + + + + Atomic Operations + + + Certain operations are guaranteed atomic on all platforms. The + first class of operations work on atomic_t + + include/asm/atomic.h; this + contains a signed integer (at least 32 bits long), and you must use + these functions to manipulate or read atomic_t variables. + atomic_read() and + atomic_set() get and set the counter, + atomic_add(), + atomic_sub(), + atomic_inc(), + atomic_dec(), and + atomic_dec_and_test() (returns + true if it was decremented to zero). + + + + Yes. It returns true (i.e. != 0) if the + atomic variable is zero. + + + + Note that these functions are slower than normal arithmetic, and + so should not be used unnecessarily. + + + + The second class of atomic operations is atomic bit operations on an + unsigned long, defined in + + include/linux/bitops.h. These + operations generally take a pointer to the bit pattern, and a bit + number: 0 is the least significant bit. + set_bit(), clear_bit() + and change_bit() set, clear, and flip the + given bit. test_and_set_bit(), + test_and_clear_bit() and + test_and_change_bit() do the same thing, + except return true if the bit was previously set; these are + particularly useful for atomically setting flags. + + + + It is possible to call these operations with bit indices greater + than BITS_PER_LONG. The resulting behavior is strange on big-endian + platforms though so it is a good idea not to do this. + + + + + Symbols + + + Within the kernel proper, the normal linking rules apply + (ie. unless a symbol is declared to be file scope with the + static keyword, it can be used anywhere in the + kernel). However, for modules, a special exported symbol table is + kept which limits the entry points to the kernel proper. Modules + can also export symbols. + + + + <function>EXPORT_SYMBOL()</function> + <filename class="headerfile">include/linux/module.h</filename> + + + This is the classic method of exporting a symbol: dynamically + loaded modules will be able to use the symbol as normal. + + + + + <function>EXPORT_SYMBOL_GPL()</function> + <filename class="headerfile">include/linux/module.h</filename> + + + Similar to EXPORT_SYMBOL() except that the + symbols exported by EXPORT_SYMBOL_GPL() can + only be seen by modules with a + MODULE_LICENSE() that specifies a GPL + compatible license. It implies that the function is considered + an internal implementation issue, and not really an interface. + + + + + + Routines and Conventions + + + Double-linked lists + <filename class="headerfile">include/linux/list.h</filename> + + + There used to be three sets of linked-list routines in the kernel + headers, but this one is the winner. If you don't have some + particular pressing need for a single list, it's a good choice. + + + + In particular, list_for_each_entry is useful. + + + + + Return Conventions + + + For code called in user context, it's very common to defy C + convention, and return 0 for success, + and a negative error number + (eg. -EFAULT) for failure. This can be + unintuitive at first, but it's fairly widespread in the kernel. + + + + Using ERR_PTR() + + include/linux/err.h; to + encode a negative error number into a pointer, and + IS_ERR() and PTR_ERR() + to get it back out again: avoids a separate pointer parameter for + the error number. Icky, but in a good way. + + + + + Breaking Compilation + + + Linus and the other developers sometimes change function or + structure names in development kernels; this is not done just to + keep everyone on their toes: it reflects a fundamental change + (eg. can no longer be called with interrupts on, or does extra + checks, or doesn't do checks which were caught before). Usually + this is accompanied by a fairly complete note to the linux-kernel + mailing list; search the archive. Simply doing a global replace + on the file usually makes things worse. + + + + + Initializing structure members + + + The preferred method of initializing structures is to use + designated initialisers, as defined by ISO C99, eg: + + +static struct block_device_operations opt_fops = { + .open = opt_open, + .release = opt_release, + .ioctl = opt_ioctl, + .check_media_change = opt_media_change, +}; + + + This makes it easy to grep for, and makes it clear which + structure fields are set. You should do this because it looks + cool. + + + + + GNU Extensions + + + GNU Extensions are explicitly allowed in the Linux kernel. + Note that some of the more complex ones are not very well + supported, due to lack of general use, but the following are + considered standard (see the GCC info page section "C + Extensions" for more details - Yes, really the info page, the + man page is only a short summary of the stuff in info). + + + + + Inline functions + + + + + Statement expressions (ie. the ({ and }) constructs). + + + + + Declaring attributes of a function / variable / type + (__attribute__) + + + + + typeof + + + + + Zero length arrays + + + + + Macro varargs + + + + + Arithmetic on void pointers + + + + + Non-Constant initializers + + + + + Assembler Instructions (not outside arch/ and include/asm/) + + + + + Function names as strings (__func__). + + + + + __builtin_constant_p() + + + + + + Be wary when using long long in the kernel, the code gcc generates for + it is horrible and worse: division and multiplication does not work + on i386 because the GCC runtime functions for it are missing from + the kernel environment. + + + + + + + C++ + + + Using C++ in the kernel is usually a bad idea, because the + kernel does not provide the necessary runtime environment + and the include files are not tested for it. It is still + possible, but not recommended. If you really want to do + this, forget about exceptions at least. + + + + + #if + + + It is generally considered cleaner to use macros in header files + (or at the top of .c files) to abstract away functions rather than + using `#if' pre-processor statements throughout the source code. + + + + + + Putting Your Stuff in the Kernel + + + In order to get your stuff into shape for official inclusion, or + even to make a neat patch, there's administrative work to be + done: + + + + + Figure out whose pond you've been pissing in. Look at the top of + the source files, inside the MAINTAINERS + file, and last of all in the CREDITS file. + You should coordinate with this person to make sure you're not + duplicating effort, or trying something that's already been + rejected. + + + + Make sure you put your name and EMail address at the top of + any files you create or mangle significantly. This is the + first place people will look when they find a bug, or when + they want to make a change. + + + + + + Usually you want a configuration option for your kernel hack. + Edit Kconfig in the appropriate directory. + The Config language is simple to use by cut and paste, and there's + complete documentation in + Documentation/kbuild/kconfig-language.txt. + + + + You may well want to make your CONFIG option only visible if + CONFIG_EXPERIMENTAL is enabled: this serves as a + warning to users. There many other fancy things you can do: see + the various Kconfig files for ideas. + + + + In your description of the option, make sure you address both the + expert user and the user who knows nothing about your feature. Mention + incompatibilities and issues here. Definitely + end your description with if in doubt, say N + (or, occasionally, `Y'); this is for people who have no + idea what you are talking about. + + + + + + Edit the Makefile: the CONFIG variables are + exported here so you can usually just add a "obj-$(CONFIG_xxx) += + xxx.o" line. The syntax is documented in + Documentation/kbuild/makefiles.txt. + + + + + + Put yourself in CREDITS if you've done + something noteworthy, usually beyond a single file (your name + should be at the top of the source files anyway). + MAINTAINERS means you want to be consulted + when changes are made to a subsystem, and hear about bugs; it + implies a more-than-passing commitment to some part of the code. + + + + + + Finally, don't forget to read Documentation/SubmittingPatches + and possibly Documentation/SubmittingDrivers. + + + + + + + Kernel Cantrips + + + Some favorites from browsing the source. Feel free to add to this + list. + + + + arch/x86/include/asm/delay.h: + + +#define ndelay(n) (__builtin_constant_p(n) ? \ + ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ + __ndelay(n)) + + + + include/linux/fs.h: + + +/* + * Kernel pointers have redundant information, so we can use a + * scheme where we can return either an error code or a dentry + * pointer with the same return value. + * + * This should be a per-architecture thing, to allow different + * error and pointer decisions. + */ + #define ERR_PTR(err) ((void *)((long)(err))) + #define PTR_ERR(ptr) ((long)(ptr)) + #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) + + + + arch/x86/include/asm/uaccess_32.h: + + + +#define copy_to_user(to,from,n) \ + (__builtin_constant_p(n) ? \ + __constant_copy_to_user((to),(from),(n)) : \ + __generic_copy_to_user((to),(from),(n))) + + + + arch/sparc/kernel/head.S: + + + +/* + * Sun people can't spell worth damn. "compatability" indeed. + * At least we *know* we can't spell, and use a spell-checker. + */ + +/* Uh, actually Linus it is I who cannot spell. Too much murky + * Sparc assembly will do this to ya. + */ +C_LABEL(cputypvar): + .asciz "compatability" + +/* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ + .align 4 +C_LABEL(cputypvar_sun4m): + .asciz "compatible" + + + + arch/sparc/lib/checksum.S: + + + + /* Sun, you just can't beat me, you just can't. Stop trying, + * give up. I'm serious, I am going to kick the living shit + * out of you, game over, lights out. + */ + + + + + Thanks + + + Thanks to Andi Kleen for the idea, answering my questions, fixing + my mistakes, filling content, etc. Philipp Rumpf for more spelling + and clarity fixes, and some excellent non-obvious points. Werner + Almesberger for giving me a great summary of + disable_irq(), and Jes Sorensen and Andrea + Arcangeli added caveats. Michael Elizabeth Chastain for checking + and adding to the Configure section. Telsa Gwynne for teaching me DocBook. + + + + -- cgit v1.2.3