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fix potential deadlock bug in libc-internal locking logic

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if a multithreaded program became non-multithreaded (i.e. all other
threads exited) while one thread held an internal lock, the remaining
thread would fail to release the lock. the the program then became
multithreaded again at a later time, any further attempts to obtain
the lock would deadlock permanently.

the underlying cause is that the value of libc.threads_minus_1 at
unlock time might not match the value at lock time. one solution would
be returning a flag to the caller indicating whether the lock was
taken and needs to be unlocked, but there is a simpler solution: using
the lock itself as such a flag.

note that this flag is not needed anyway for correctness; if the lock
is not held, the unlock code is harmless. however, the memory
synchronization properties associated with a_store are costly on some
archs, so it's best to avoid executing the unlock code when it is
unnecessary.
This commit is contained in:
Rich Felker 2013-09-20 02:00:27 -04:00
commit e803829e6b
3 changed files with 15 additions and 13 deletions
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9
src/thread/__lock.c
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@ -2,11 +2,14 @@
void __lock(volatile int *l)
{
while (a_swap(l, 1)) __wait(l, l+1, 1, 1);
if (libc.threads_minus_1)
while (a_swap(l, 1)) __wait(l, l+1, 1, 1);
}
void __unlock(volatile int *l)
{
a_store(l, 0);
if (l[1]) __wake(l, 1, 1);
if (l[0]) {
a_store(l, 0);
if (l[1]) __wake(l, 1, 1);
}
}