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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.
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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.
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new features:
- FUTEX_WAIT_BITSET op will be used for timed waits if available. this
saves a call to clock_gettime.
- error checking for the timespec struct is now inside __timedwait so
it doesn't need to be duplicated everywhere. cond_timedwait still
needs to duplicate it to avoid unlocking the mutex, though.
- pushing and popping the cancellation handler is delegated to
__timedwait, and cancellable/non-cancellable waits are unified.
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this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.
the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.
these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.
x86_64 asm is untested and may need a second try to get it right.
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