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to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
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this dec used to be performed by the cancellation handler, which was
called when popped.
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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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the race condition these changes address is described in glibc bug
report number 12674:
http://sourceware.org/bugzilla/show_bug.cgi?id=12674
up until now, musl has shared the bug, and i had not been able to
figure out how to eliminate it. in short, the problem is that it's not
valid for sem_post to inspect the waiters count after incrementing the
semaphore value, because another thread may have already successfully
returned from sem_wait, (rightly) deemed itself the only remaining
user of the semaphore, and chosen to destroy and free it (or unmap the
shared memory it's stored in). POSIX is not explicit in blessing this
usage, but it gives a very explicit analogous example with mutexes
(which, in musl and glibc, also suffer from the same race condition
bug) in the rationale for pthread_mutex_destroy.
the new semaphore implementation augments the waiter count with a
redundant waiter indication in the semaphore value itself,
representing the presence of "last minute" waiters that may have
arrived after sem_post read the waiter count. this allows sem_post to
read the waiter count prior to incrementing the semaphore value,
rather than after incrementing it, so as to avoid accessing the
semaphore memory whatsoever after the increment takes place.
a similar, but much simpler, fix should be possible for mutexes and
other locking primitives whose usage rules are stricter than
semaphores.
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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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1. make sem_[timed]wait interruptible by signals, per POSIX
2. keep a waiter count in order to avoid unnecessary futex wake syscalls
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this commit addresses two issues:
1. a race condition, whereby a cancellation request occurring after a
syscall returned from kernelspace but before the subsequent
CANCELPT_END would cause cancellable resource-allocating syscalls
(like open) to leak resources.
2. signal handlers invoked while the thread was blocked at a
cancellation point behaved as if asynchronous cancellation mode wer in
effect, resulting in potentially dangerous state corruption if a
cancellation request occurs.
the glibc/nptl implementation of threads shares both of these issues.
with this commit, both are fixed. however, cancellation points
encountered in a signal handler will not be acted upon if the signal
was received while the thread was already at a cancellation point.
they will of course be acted upon after the signal handler returns, so
in real-world usage where signal handlers quickly return, it should
not be a problem. it's possible to solve this problem too by having
sigaction() wrap all signal handlers with a function that uses a
pthread_cleanup handler to catch cancellation, patch up the saved
context, and return into the cancellable function that will catch and
act upon the cancellation. however that would be a lot of complexity
for minimal if any benefit...
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