path: root/src
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authorRich Felker <>2015-02-03 00:16:55 -0500
committerRich Felker <>2015-02-03 00:16:55 -0500
commit3559f0b894e4369955f8467b94cb8fda537d2f36 (patch)
tree1125e45cfb86040484b9923c84a158914ac23760 /src
parent4758f0565df83beb8f9b24857ec40ca3a40d2281 (diff)
fix missing memory barrier in cancellation signal handler
in practice this was probably a non-issue, because the necessary barrier almost certainly exists in kernel space -- implementing signal delivery without such a barrier seems impossible -- but for the sake of correctness, it should be done here too. in principle, without a barrier, it is possible that the thread to be cancelled does not see the store of its cancellation flag performed by another thread. this affects both the case where the signal arrives before entering the critical program counter range from __cp_begin to __cp_end (in which case both the signal handler and the inline check fail to see the value which was already stored) and the case where the signal arrives during the critical range (in which case the signal handler should be responsible for cancellation, but when it does not see the cancellation flag, it assumes the signal is spurious and refuses to act on it). in the fix, the barrier is placed only in the signal handler, not in the inline check at the beginning of the critical program counter range. if the signal handler runs before the critical range is entered, it will of course take no action, but its barrier will ensure that the inline check subsequently sees the store. if on the other hand the inline check runs first, it may miss seeing the store, but the subsequent signal handler in the critical range will act upon the cancellation request. this strategy avoids adding a memory barrier in the common, non-cancellation code path.
Diffstat (limited to 'src')
1 files changed, 1 insertions, 0 deletions
diff --git a/src/thread/pthread_cancel.c b/src/thread/pthread_cancel.c
index e3d291ee..f17ce882 100644
--- a/src/thread/pthread_cancel.c
+++ b/src/thread/pthread_cancel.c
@@ -42,6 +42,7 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
const char *ip = ((char **)&uc->uc_mcontext)[CANCEL_REG_IP];
extern const char __cp_begin[1], __cp_end[1];
+ a_barrier();
if (!self->cancel || self->canceldisable) return;
_sigaddset(&uc->uc_sigmask, SIGCANCEL);