e803829e6b
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.
15 lines
230 B
C
15 lines
230 B
C
#include "pthread_impl.h"
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void __lock(volatile int *l)
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{
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if (libc.threads_minus_1)
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while (a_swap(l, 1)) __wait(l, l+1, 1, 1);
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}
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void __unlock(volatile int *l)
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{
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if (l[0]) {
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a_store(l, 0);
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if (l[1]) __wake(l, 1, 1);
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}
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}
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