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diff --git a/Documentation/robust-futexes.txt b/Documentation/robust-futexes.txt new file mode 100644 index 00000000..0a9446a5 --- /dev/null +++ b/Documentation/robust-futexes.txt @@ -0,0 +1,218 @@ +Started by: Ingo Molnar <mingo@redhat.com> + +Background +---------- + +what are robust futexes? To answer that, we first need to understand +what futexes are: normal futexes are special types of locks that in the +noncontended case can be acquired/released from userspace without having +to enter the kernel. + +A futex is in essence a user-space address, e.g. a 32-bit lock variable +field. If userspace notices contention (the lock is already owned and +someone else wants to grab it too) then the lock is marked with a value +that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT) +syscall is used to wait for the other guy to release it. The kernel +creates a 'futex queue' internally, so that it can later on match up the +waiter with the waker - without them having to know about each other. +When the owner thread releases the futex, it notices (via the variable +value) that there were waiter(s) pending, and does the +sys_futex(FUTEX_WAKE) syscall to wake them up. Once all waiters have +taken and released the lock, the futex is again back to 'uncontended' +state, and there's no in-kernel state associated with it. The kernel +completely forgets that there ever was a futex at that address. This +method makes futexes very lightweight and scalable. + +"Robustness" is about dealing with crashes while holding a lock: if a +process exits prematurely while holding a pthread_mutex_t lock that is +also shared with some other process (e.g. yum segfaults while holding a +pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need +to be notified that the last owner of the lock exited in some irregular +way. + +To solve such types of problems, "robust mutex" userspace APIs were +created: pthread_mutex_lock() returns an error value if the owner exits +prematurely - and the new owner can decide whether the data protected by +the lock can be recovered safely. + +There is a big conceptual problem with futex based mutexes though: it is +the kernel that destroys the owner task (e.g. due to a SEGFAULT), but +the kernel cannot help with the cleanup: if there is no 'futex queue' +(and in most cases there is none, futexes being fast lightweight locks) +then the kernel has no information to clean up after the held lock! +Userspace has no chance to clean up after the lock either - userspace is +the one that crashes, so it has no opportunity to clean up. Catch-22. + +In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot +is needed to release that futex based lock. This is one of the leading +bugreports against yum. + +To solve this problem, the traditional approach was to extend the vma +(virtual memory area descriptor) concept to have a notion of 'pending +robust futexes attached to this area'. This approach requires 3 new +syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and +FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether +they have a robust_head set. This approach has two fundamental problems +left: + + - it has quite complex locking and race scenarios. The vma-based + approach had been pending for years, but they are still not completely + reliable. + + - they have to scan _every_ vma at sys_exit() time, per thread! + +The second disadvantage is a real killer: pthread_exit() takes around 1 +microsecond on Linux, but with thousands (or tens of thousands) of vmas +every pthread_exit() takes a millisecond or more, also totally +destroying the CPU's L1 and L2 caches! + +This is very much noticeable even for normal process sys_exit_group() +calls: the kernel has to do the vma scanning unconditionally! (this is +because the kernel has no knowledge about how many robust futexes there +are to be cleaned up, because a robust futex might have been registered +in another task, and the futex variable might have been simply mmap()-ed +into this process's address space). + +This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that +normal kernels can turn it off, but worse than that: the overhead makes +robust futexes impractical for any type of generic Linux distribution. + +So something had to be done. + +New approach to robust futexes +------------------------------ + +At the heart of this new approach there is a per-thread private list of +robust locks that userspace is holding (maintained by glibc) - which +userspace list is registered with the kernel via a new syscall [this +registration happens at most once per thread lifetime]. At do_exit() +time, the kernel checks this user-space list: are there any robust futex +locks to be cleaned up? + +In the common case, at do_exit() time, there is no list registered, so +the cost of robust futexes is just a simple current->robust_list != NULL +comparison. If the thread has registered a list, then normally the list +is empty. If the thread/process crashed or terminated in some incorrect +way then the list might be non-empty: in this case the kernel carefully +walks the list [not trusting it], and marks all locks that are owned by +this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if +any). + +The list is guaranteed to be private and per-thread at do_exit() time, +so it can be accessed by the kernel in a lockless way. + +There is one race possible though: since adding to and removing from the +list is done after the futex is acquired by glibc, there is a few +instructions window for the thread (or process) to die there, leaving +the futex hung. To protect against this possibility, userspace (glibc) +also maintains a simple per-thread 'list_op_pending' field, to allow the +kernel to clean up if the thread dies after acquiring the lock, but just +before it could have added itself to the list. Glibc sets this +list_op_pending field before it tries to acquire the futex, and clears +it after the list-add (or list-remove) has finished. + +That's all that is needed - all the rest of robust-futex cleanup is done +in userspace [just like with the previous patches]. + +Ulrich Drepper has implemented the necessary glibc support for this new +mechanism, which fully enables robust mutexes. + +Key differences of this userspace-list based approach, compared to the +vma based method: + + - it's much, much faster: at thread exit time, there's no need to loop + over every vma (!), which the VM-based method has to do. Only a very + simple 'is the list empty' op is done. + + - no VM changes are needed - 'struct address_space' is left alone. + + - no registration of individual locks is needed: robust mutexes dont + need any extra per-lock syscalls. Robust mutexes thus become a very + lightweight primitive - so they dont force the application designer + to do a hard choice between performance and robustness - robust + mutexes are just as fast. + + - no per-lock kernel allocation happens. + + - no resource limits are needed. + + - no kernel-space recovery call (FUTEX_RECOVER) is needed. + + - the implementation and the locking is "obvious", and there are no + interactions with the VM. + +Performance +----------- + +I have benchmarked the time needed for the kernel to process a list of 1 +million (!) held locks, using the new method [on a 2GHz CPU]: + + - with FUTEX_WAIT set [contended mutex]: 130 msecs + - without FUTEX_WAIT set [uncontended mutex]: 30 msecs + +I have also measured an approach where glibc does the lock notification +[which it currently does for !pshared robust mutexes], and that took 256 +msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls +userspace had to do. + +(1 million held locks are unheard of - we expect at most a handful of +locks to be held at a time. Nevertheless it's nice to know that this +approach scales nicely.) + +Implementation details +---------------------- + +The patch adds two new syscalls: one to register the userspace list, and +one to query the registered list pointer: + + asmlinkage long + sys_set_robust_list(struct robust_list_head __user *head, + size_t len); + + asmlinkage long + sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, + size_t __user *len_ptr); + +List registration is very fast: the pointer is simply stored in +current->robust_list. [Note that in the future, if robust futexes become +widespread, we could extend sys_clone() to register a robust-list head +for new threads, without the need of another syscall.] + +So there is virtually zero overhead for tasks not using robust futexes, +and even for robust futex users, there is only one extra syscall per +thread lifetime, and the cleanup operation, if it happens, is fast and +straightforward. The kernel doesn't have any internal distinction between +robust and normal futexes. + +If a futex is found to be held at exit time, the kernel sets the +following bit of the futex word: + + #define FUTEX_OWNER_DIED 0x40000000 + +and wakes up the next futex waiter (if any). User-space does the rest of +the cleanup. + +Otherwise, robust futexes are acquired by glibc by putting the TID into +the futex field atomically. Waiters set the FUTEX_WAITERS bit: + + #define FUTEX_WAITERS 0x80000000 + +and the remaining bits are for the TID. + +Testing, architecture support +----------------------------- + +i've tested the new syscalls on x86 and x86_64, and have made sure the +parsing of the userspace list is robust [ ;-) ] even if the list is +deliberately corrupted. + +i386 and x86_64 syscalls are wired up at the moment, and Ulrich has +tested the new glibc code (on x86_64 and i386), and it works for his +robust-mutex testcases. + +All other architectures should build just fine too - but they wont have +the new syscalls yet. + +Architectures need to implement the new futex_atomic_cmpxchg_inatomic() +inline function before writing up the syscalls (that function returns +-ENOSYS right now). |