fix async-cancel-safety of pthread_cancel

the previous commit addressing async-signal-safety issues around
pthread_kill did not fully fix pthread_cancel, which is also required
(albeit rather irrationally) to be async-cancel-safe.

without blocking implementation-internal signals, it's possible that,
when async cancellation is enabled, a cancel signal sent by another
thread interrupts pthread_kill while the killlock for a targeted
thread is held. as a result, the calling thread will terminate due to
cancellation without ever unlocking the targeted thread's killlock,
and thus the targeted thread will be unable to exit.

diff --git a/src/thread/pthread_kill.c b/src/thread/pthread_kill.c
index 446254b..79ddb20 100644 (file)
--- a/src/thread/pthread_kill.c
+++ b/src/thread/pthread_kill.c
@@ -5,7 +5,10 @@ int pthread_kill(pthread_t t, int sig)
 {
        int r;
        sigset_t set;
-       __block_app_sigs(&set);
+       /* Block not just app signals, but internal ones too, since
+        * pthread_kill is used to implement pthread_cancel, which
+        * must be async-cancel-safe. */
+       __block_all_sigs(&set);
        LOCK(t->killlock);
        r = t->tid ? -__syscall(SYS_tkill, t->tid, sig)
                : (sig+0U >= _NSIG ? EINVAL : 0);