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* ditch the priority inheritance locks; use malloc's version of lockRich Felker2012-04-242-28/+8
| | | | | | | | | | | | | | | | | | | i did some testing trying to switch malloc to use the new internal lock with priority inheritance, and my malloc contention test got 20-100 times slower. if priority inheritance futexes are this slow, it's simply too high a price to pay for avoiding priority inversion. maybe we can consider them somewhere down the road once the kernel folks get their act together on this (and perferably don't link it to glibc's inefficient lock API)... as such, i've switch __lock to use malloc's implementation of lightweight locks, and updated all the users of the code to use an array with a waiter count for their locks. this should give optimal performance in the vast majority of cases, and it's simple. malloc is still using its own internal copy of the lock code because it seems to yield measurably better performance with -O3 when it's inlined (20% or more difference in the contention stress test).
* internal locks: new owner of contended lock must set waiters flagRich Felker2012-04-241-1/+1
| | | | | | | | | | | | | | | | | | | | | | this bug probably would have gone unnoticed since it's only used in the fallback code for systems where priority-inheritance locking fails. unfortunately this approach results in one spurious wake syscall on the final unlock, when there are no waiters remaining. the alternative (possibly better) would be to use broadcast wakes instead of reflagging the waiter unconditionally, and let each waiter reflag itself; this saves one syscall at the expense of invoking the "thundering herd" effect (worse performance degredation) when there are many waiters. ideally we would be able to update all of our locks to use an array of two ints rather than a single int, and use a separate counter system like proper mutexes use; then we could avoid all spurious wake calls without resorting to broadcasts. however, it's not clear to me that priority inheritance futexes support this usage. the kernel sets the waiters flag for them (just like we're doing now) and i can't tell if it's safe to bypass the kernel when unlocking just because we know (from private data, the waiter count) that there are no waiters. this is something that could be explored in the future.
* new internal locking primitive; drop spinlocksRich Felker2012-04-241-6/+27
| | | | | | we use priority inheritance futexes if possible so that the library cannot hit internal priority inversion deadlocks in the presence of realtime priority scheduling (full support to be added later).
* fix pthread_cleanup_pop(1) crash in non-thread-capable, static-linked programsRich Felker2012-02-282-2/+2
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* work around "signal loses thread pointer" issue with "approach 2"Rich Felker2012-02-271-2/+3
| | | | | | | | | | | | this was discussed on the mailing list and no consensus on the preferred solution was reached, so in anticipation of a release, i'm just committing a minimally-invasive solution that avoids the problem by ensuring that multi-threaded-capable programs will always have initialized the thread pointer before any signal handler can run. in the long term we may switch to initializing the thread pointer at program start time whenever the program has the potential to access any per-thread data.
* small fix for new pthread cleanup stuffRich Felker2012-02-092-1/+1
| | | | | | | even if pthread_create/exit code is not linked, run flag needs to be checked and cleanup function potentially run on pop. thus, move the code to the module that's always linked when pthread_cleanup_push/pop is used.
* replace bad cancellation cleanup abi with a sane oneRich Felker2012-02-093-62/+23
| | | | | | | | | | | | | | | | | | | | | | the old abi was intended to duplicate glibc's abi at the expense of being ugly and slow, but it turns out glib was not even using that abi except on non-gcc-compatible compilers (which it doesn't even support) and was instead using an exceptions-in-c/unwind-based approach whose abi we could not duplicate anyway without nasty dwarf2/unwind integration. the new abi is copied from a very old glibc abi, which seems to still be supported/present in current glibc. it avoids all unwinding, whether by sjlj or exceptions, and merely maintains a linked list of cleanup functions to be called from the context of pthread_exit. i've made some care to ensure that longjmp out of a cleanup function should work, even though it is not required to. this change breaks abi compatibility with programs which were using pthread cancellation, which is unfortunate, but that's why i'm making the change now rather than later. considering that most pthread features have not been usable until recently anyway, i don't see it as a major issue at this point.
* fix cancellation failure in single-threaded programsRich Felker2012-01-221-1/+1
| | | | | | | even a single-threaded program can be cancellable, e.g. if it's called pthread_cancel(pthread_self()). the correct predicate to check is not whether multiple threads have been invoked, but whether pthread_self has been invoked.
* report sem value overflows in sem_postRich Felker2011-10-261-0/+4
| | | | | this is not required by the standard, but it's nicer than corrupting the state and rather inexpensive.
* fix typo in arm clone() asmRich Felker2011-10-091-1/+1
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* recovering ownerdead robust mutex must reset recursive lock countRich Felker2011-10-031-0/+1
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* simplify robust mutex unlock code pathRich Felker2011-10-031-4/+4
| | | | | | | right now it's questionable whether this change is an improvement or not, but if we later want to support priority inheritance mutexes, it will be important to have the code paths unified like this to avoid major code duplication.
* fix crash if pthread_mutex_unlock is called without ever lockingRich Felker2011-10-031-1/+1
| | | | | this is valid for error-checking mutexes; otherwise it invokes UB and would be justified in crashing.
* use count=0 instead of 1 for recursive mutex with only one lock referenceRich Felker2011-10-032-4/+2
| | | | | | | this simplifies the code paths slightly, but perhaps what's nicer is that it makes recursive mutexes fully reentrant, i.e. locking and unlocking from a signal handler works even if the interrupted code was in the middle of locking or unlocking.
* synchronize cond var destruction with exiting waitsRich Felker2011-10-022-0/+11
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* fix failure-to-wake in rwlock unlockRich Felker2011-10-011-1/+1
| | | | | | | | | a reader unlocking the lock need only wake one waiter (necessarily a writer, but a writer unlocking the lock must wake all waiters (necessarily readers). if it only wakes one, the remainder can remain blocked indefinitely, or at least until the first reader unlocks (in which case the whole lock becomes serialized and behaves as a mutex rather than a read lock).
* fix excessive/insufficient wakes in __vm_unlockRich Felker2011-09-281-3/+3
| | | | | | there is no need to send a wake when the lock count does not hit zero, but when it does, all waiters must be woken (since all with the same sign are eligible to obtain the lock).
* improve pshared barriersRich Felker2011-09-281-11/+13
| | | | | | | | | | | | | | eliminate the sequence number field and instead use the counter as the futex because of the way the lock is held, sequence numbers are completely useless, and this frees up a field in the barrier structure to be used as a waiter count for the count futex, which lets us avoid some syscalls in the best case. as of now, self-synchronized destruction and unmapping should be fully safe. before any thread can return from the barrier, all threads in the barrier have obtained the vm lock, and each holds a shared lock on the barrier. the barrier memory is not inspected after the shared lock count reaches 0, nor after the vm lock is released.
* next step making barrier self-sync'd destruction safeRich Felker2011-09-282-6/+18
| | | | i think this works, but it can be simplified. (next step)
* barrier destroy must also wait for threads in other processes exiting barrierRich Felker2011-09-281-0/+2
| | | | | the vm lock only waits for threads in the same process exiting. actually this fix is not enough, but it's a start...
* correctly handle the degenerate barrier in the pshared caseRich Felker2011-09-271-1/+1
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* fix crash in pthread_cond_wait mutex-locked checkRich Felker2011-09-271-1/+1
| | | | | | it was assuming the result of the condition it was supposed to be checking for, i.e. that the thread ptr had already been initialized by pthread_mutex_lock. use the slower call to be safe.
* fix crash in pthread_testcancel if pthread_self has not been calledRich Felker2011-09-271-1/+1
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* improve/debloat mutex unlock error checking in pthread_cond_waitRich Felker2011-09-271-3/+3
| | | | | | | we're not required to check this except for error-checking mutexes, but it doesn't hurt. the new test is actually simpler/lighter, and it also eliminates the need to later check that pthread_mutex_unlock succeeds.
* check mutex owner in pthread_cond_waitRich Felker2011-09-271-0/+3
| | | | | | | | | | when used with error-checking mutexes, pthread_cond_wait is required to fail with EPERM if the mutex is not locked by the caller. previously we relied on pthread_mutex_unlock to generate the error, but this is not valid, since in the case of such invalid usage the internal state of the cond variable has already been potentially corrupted (due to access outside the control of the mutex). thus, we have to check first.
* fix pshared barrier wrong return value.Rich Felker2011-09-271-1/+1
| | | | i set the return value but then never used it... oops!
* convert the barrier pshared option back to 0/1 values when getting itRich Felker2011-09-271-1/+1
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* process-shared barrier support, based on discussion with bdonlanRich Felker2011-09-274-10/+76
| | | | | | | | | | | | | this implementation is rather heavy-weight, but it's the first solution i've found that's actually correct. all waiters actually wait twice at the barrier so that they can synchronize exit, and they hold a "vm lock" that prevents changes to virtual memory mappings (and blocks pthread_barrier_destroy) until all waiters are finished inspecting the barrier. thus, it is safe for any thread to destroy and/or unmap the barrier's memory as soon as pthread_barrier_wait returns, without further synchronization.
* fix incorrect allocation failure check in pthread_createRich Felker2011-09-271-1/+1
| | | | | mmap returns MAP_FAILED not 0 because some idiot thought the ability to mmap the null pointer page would be a good idea...
* another cond var fix: requeue count race conditionRich Felker2011-09-262-15/+13
| | | | | | | | | | | | | lock out new waiters during the broadcast. otherwise the wait count added to the mutex might be lower than the actual number of waiters moved, and wakeups may be lost. this issue could also be solved by temporarily setting the mutex waiter count higher than any possible real count, then relying on the kernel to tell us how many waiters were requeued, and updating the counts afterwards. however the logic is more complex, and i don't really trust the kernel. the solution here is also nice in that it replaces some atomic cas loops with simple non-atomic ops under lock.
* fix lost signals in cond varsRich Felker2011-09-262-14/+16
| | | | | | | | | | | due to moving waiters from the cond var to the mutex in bcast, these waiters upon wakeup would steal slots in the count from newer waiters that had not yet been signaled, preventing the signal function from taking any action. to solve the problem, we simply use two separate waiter counts, and so that the original "total" waiters count is undisturbed by broadcast and still available for signal.
* redo cond vars again, use sequence numbersRich Felker2011-09-263-45/+49
| | | | | | | | | | | | | | | | | | | | | | | | testing revealed that the old implementation, while correct, was giving way too many spurious wakeups due to races changing the value of the condition futex. in a test program with 5 threads receiving broadcast signals, the number of returns from pthread_cond_wait was roughly 3 times what it should have been (2 spurious wakeups for every legitimate wakeup). moreover, the magnitude of this effect seems to grow with the number of threads. the old implementation may also have had some nasty race conditions with reuse of the cond var with a new mutex. the new implementation is based on incrementing a sequence number with each signal event. this sequence number has nothing to do with the number of threads intended to be woken; it's only used to provide a value for the futex wait to avoid deadlock. in theory there is a danger of race conditions due to the value wrapping around after 2^32 signals. it would be nice to eliminate that, if there's a way. testing showed no spurious wakeups (though they are of course possible) with the new implementation, as well as slightly improved performance.
* revert previous change in cond var waiter moveRich Felker2011-09-251-2/+6
| | | | | | using swap has a race condition: the waiters must be added to the mutex waiter count *before* they are taken off the cond var waiter count, or wake events can be lost.
* optimize cond waiter move using atomic swap instead of cas loopRich Felker2011-09-251-6/+2
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* fix logic for when wakeup is not desired on cond bcastRich Felker2011-09-251-3/+4
| | | | somehow i forgot that normal-type mutexes don't store the owner tid.
* new futex-requeue-based pthread_cond_broadcast implementationRich Felker2011-09-253-4/+63
| | | | | | this avoids the "stampede effect" where pthread_cond_broadcast would result in all waiters waking up simultaneously, only to immediately contend for the mutex and go back to sleep.
* fix ABA race in cond vars, improve them overallRich Felker2011-09-233-11/+12
| | | | | | | | | | | | | | | | | | | | | previously, a waiter could miss the 1->0 transition of block if another thread set block to 1 again after the signal function set block to 0. we now use the caller's thread id as a unique token to store in block, which no other thread will ever write there. this ensures that if block still contains the tid, no signal has occurred. spurious wakeups will of course occur whenever there is a spurious return from the futex wait and another thread has begun waiting on the cond var. this should be a rare occurrence except perhaps in the presence of interrupting signal handlers. signal/bcast operations have been improved by noting that they need not avoid inspecting the cond var's memory after changing the futex value. because the standard allows spurious wakeups, there is no way for an application to distinguish between a spurious wakeup just before another thread called signal/bcast, and the deliberate wakeup resulting from the signal/bcast call. thus the woken thread must assume that the signalling thread may still be waiting to act on the cond var, and therefore it cannot destroy/unmap the cond var.
* fix deadlock in condition wait whenever there are multiple waitersRich Felker2011-09-223-5/+17
| | | | | it's amazing none of the conformance tests i've run even bothered to check whether something so basic works...
* initial commit of the arm portRich Felker2011-09-184-0/+78
| | | | | | | | | | | | | this port assumes eabi calling conventions, eabi linux syscall convention, and presence of the kernel helpers at 0xffff0f?0 needed for threads support. otherwise it makes very few assumptions, and the code should work even on armv4 without thumb support, as well as on systems with thumb interworking. the bits headers declare this a little endian system, but as far as i can tell the code should work equally well on big endian. some small details are probably broken; so far, testing has been limited to qemu/aboriginal linux.
* overhaul clone syscall wrappingRich Felker2011-09-187-60/+107
| | | | | | | | | | | | | | | | | | | | several things are changed. first, i have removed the old __uniclone function signature and replaced it with the "standard" linux __clone/clone signature. this was necessary to expose clone to applications anyway, and it makes it easier to port __clone to new archs, since it's now testable independently of pthread_create. secondly, i have removed all references to the ugly ldt descriptor structure (i386 only) from the c code and pthread structure. in places where it is needed, it is now created on the stack just when it's needed, in assembly code. thus, the i386 __clone function takes the desired thread pointer as its argument, rather than an ldt descriptor pointer, just like on all other sane archs. this should not affect applications since there is really no way an application can use clone with threads/tls in a way that doesn't horribly conflict with and clobber the underlying implementation's use. applications are expected to use clone only for creating actual processes, possibly with new namespace features and whatnot.
* dummy implementation of set_thread_areaRich Felker2011-09-171-0/+6
| | | | | | | | eventually we may have a working "generic" implementation for archs that don't need anything special. in any case, the goal of having stubs like this is to allow early testing of new ports before all the details needed for threads have been filled in. more functions like this will follow.
* fix more instances of old a_xchg (use new a_swap name)Rich Felker2011-09-163-3/+3
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* use a_swap rather than old name a_xchgRich Felker2011-09-161-1/+1
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* remove some stray trailing space charactersRich Felker2011-09-131-1/+1
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* fix serious bug in pthread_joinRich Felker2011-09-111-2/+2
| | | | | | | | | | | | | on spurious wakeups/returns from __timedwait, pthread_join would "succeed" and unmap the thread's stack while it was still running. at best this would lead to SIGSEGV when the thread resumed execution, but in the worst case, the thread would later resume executing on top of another new thread's stack mapped at the same address. spent about 4 hours tracking this bug down, chasing rare difficult-to-reproduce stack corruption in a stress test program. still no idea *what* caused the spurious wakeups; i suspect it's a kernel bug.
* fix pthread_join wait call: thread termination tid futex is not privateRich Felker2011-09-091-1/+1
| | | | | | | | | this seeme to be the bug that prevented enabling of private futex support. i'm going to hold off on switching to private futexes until after the next release, and until i get a chance to audit all wait/wake calls to make sure they're using the correct private argument, but with this change it should be safe to enable private futex support.
* handle pending cancellation when enabling async cancellationRich Felker2011-09-041-0/+1
| | | | | | | | | | | this is not strictly required by the standard, but without it, there is a race condition where cancellation arriving just before async cancellation is enabled might not be acted upon. it is impossible for a conforming application to work around this race condition since calling pthread_testcancel after setting async cancellation mode is not allowed (pthread_testcancel is not specified to be async-cancel-safe). thus the implementation should be responsible for eliminating the race, from a quality-of-implementation standpoint.
* macro for pthread_equalRich Felker2011-08-141-1/+1
| | | | no sense bloating apps with a function call for an equality comparison...
* implement forkallRich Felker2011-08-121-0/+66
| | | | | | | | | | | | this is a "nonstandard" function that was "rejected" by POSIX, but nonetheless had its behavior documented in the POSIX rationale for fork. it's present on solaris and possibly some other systems, and duplicates the whole calling process, not just a single thread. glibc does not have this function. it should not be used in programs intending to be portable, but may be useful for testing, checkpointing, etc. and it's an interesting (and quite small) example of the usefulness of the __synccall framework originally written to work around deficiencies in linux's setuid syscall.
* pthread and synccall cleanup, new __synccall_wait opRich Felker2011-08-122-7/+11
| | | | | | | | | fix up clone signature to match the actual behavior. the new __syncall_wait function allows a __synccall callback to wait for other threads to continue without returning, so that it can resume action after the caller finishes. this interface could be made significantly more general/powerful with minimal effort, but i'll wait to do that until it's actually useful for something.
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