- 29 9月, 2011 5 次提交
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由 Rich Felker 提交于
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).
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由 Rich Felker 提交于
seeking back can be performed by the caller, but if the caller doesn't expect it, it will result in an infinite loop of failures.
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
i think this works, but it can be simplified. (next step)
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由 Rich Felker 提交于
the vm lock only waits for threads in the same process exiting. actually this fix is not enough, but it's a start...
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- 28 9月, 2011 13 次提交
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
i set the return value but then never used it... oops!
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由 Rich Felker 提交于
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由 Rich Felker 提交于
not sure if this is correct/ideal. it needs further attention.
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
mmap returns MAP_FAILED not 0 because some idiot thought the ability to mmap the null pointer page would be a good idea...
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- 27 9月, 2011 3 次提交
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由 Rich Felker 提交于
issue reported by nsz, but it's actually not just pedantic. the functions can take input of any arithmetic type, including floating point, and the behavior needs to be as if the conversion implicit in the function call took place.
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
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.
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- 26 9月, 2011 4 次提交
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由 Rich Felker 提交于
the changes to syscall_ret are mostly no-ops in the generated code, just cleanup of type issues and removal of some implementation-defined behavior. the one exception is the change in the comparison value, which is fixed so that 0xf...f000 (which in principle could be a valid return value for mmap, although probably never in reality) is not treated as an error return.
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
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- 25 9月, 2011 2 次提交
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由 Rich Felker 提交于
somehow i forgot that normal-type mutexes don't store the owner tid.
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由 Rich Felker 提交于
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.
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- 24 9月, 2011 3 次提交
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由 Rich Felker 提交于
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.
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由 Rich Felker 提交于
casting to int would not be correct because high bits could be lost. mapping the high bits down onto low bits would be costlier in the common case where the result is just used in a conditional. changing the type of the bit array elements to int would permute the order of the bit array on 64-bit big endian systems, so that's not an option either.
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由 Rich Felker 提交于
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- 23 9月, 2011 5 次提交
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由 Rich Felker 提交于
actually this is just to avoid gcc being stupid and refusing to inline the function version, even when the size cost is essentially identical whether it's inlined or not.
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由 Rich Felker 提交于
it's amazing none of the conformance tests i've run even bothered to check whether something so basic works...
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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- 22 9月, 2011 5 次提交
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由 Rich Felker 提交于
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由 Rich Felker 提交于
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由 Rich Felker 提交于
if the file descriptor resource limit has been increased past FD_SETSIZE, this is actually a security issue; we could write past the end of the fd_set object. using poll makes it a non-issue, and simplifies the code at the same time. also, use clock_gettime instead of gettimeofday, for reduced bloat and better entropy.
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由 Rich Felker 提交于
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由 Rich Felker 提交于
for now this is just a tiny optimization, but later if we support cancellation from __stdio_read and __stdio_write, it will be necessary for the recusrive lock count to be zero in order for these functions to know they are responsible for unlocking the FILE on cancellation.
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