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.
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.
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.
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.
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.
