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* another cond var fix: requeue count race conditionRich Felker2011-09-261-6/+2
| | | | | | | | | | | | | 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 Felker2011-09-261-2/+6
| | | | | | | | | | | 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 Felker2011-09-261-27/+18
| | | | | | | | | | | | | | | | | | | | | | | | 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 Felker2011-09-251-1/+40
| | | | | | 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 Felker2011-09-231-3/+2
| | | | | | | | | | | | | | | | | | | | | 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 Felker2011-09-221-1/+2
| | | | | it's amazing none of the conformance tests i've run even bothered to check whether something so basic works...
* condition variable signal/bcast need not wake unless there are waitersRich Felker2011-08-071-2/+2
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* reorganize pthread data structures and move the definitions to alltypes.hRich Felker2011-02-171-2/+2
| | | | | | | | this allows sys/types.h to provide the pthread types, as required by POSIX. this design also facilitates forcing ABI-compatible sizes in the arch-specific alltypes.h, while eliminating the need for developers changing the internals of the pthread types to poke around with arch-specific headers they may not be able to test.
* initial check-in, version 0.5.0 v0.5.0Rich Felker2011-02-121-0/+8