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- commit da767eaaf6128eccd349a54ec6eac2a68dcacacb
- Author: Willy Tarreau <w@1wt.eu>
- Date: Wed Jul 31 19:15:45 2019 +0200
-
- BUG/MINOR: debug: fix a small race in the thread dumping code
-
- If a thread dump is requested from a signal handler, it may interrupt
- a thread already waiting for a dump to complete, and may see the
- threads_to_dump variable go to zero while others are waiting, steal
- the lock and prevent other threads from ever completing. This tends
- to happen when dumping many threads upon a watchdog timeout, to threads
- waiting for their turn.
-
- Instead now we proceed in two steps :
- 1) the last dumped thread sets all bits again
- 2) all threads only wait for their own bit to appear, then clear it
- and quit
-
- This way there's no risk that a bit performs a double flip in the same
- loop and threads cannot get stuck here anymore.
-
- This should be backported to 2.0 as it clarifies stack traces.
-
- (cherry picked from commit c07736209db764fb2aef6f18ed3687a504c35771)
- Signed-off-by: Willy Tarreau <w@1wt.eu>
-
- diff --git a/src/debug.c b/src/debug.c
- index 059bc6b9..07624ca5 100644
- --- a/src/debug.c
- +++ b/src/debug.c
- @@ -440,8 +440,8 @@ void debug_handler(int sig, siginfo_t *si, void *arg)
- * 1- wait for our turn, i.e. when all lower bits are gone.
- * 2- perform the action if our bit is set
- * 3- remove our bit to let the next one go, unless we're
- - * the last one and have to put them all but ours
- - * 4- wait for zero and clear our bit if it's set
- + * the last one and have to put them all as a signal
- + * 4- wait out bit to re-appear, then clear it and quit.
- */
-
- /* wait for all previous threads to finish first */
- @@ -454,7 +454,7 @@ void debug_handler(int sig, siginfo_t *si, void *arg)
- ha_thread_dump(thread_dump_buffer, tid, thread_dump_tid);
- if ((threads_to_dump & all_threads_mask) == tid_bit) {
- /* last one */
- - HA_ATOMIC_STORE(&threads_to_dump, all_threads_mask & ~tid_bit);
- + HA_ATOMIC_STORE(&threads_to_dump, all_threads_mask);
- thread_dump_buffer = NULL;
- }
- else
- @@ -462,14 +462,13 @@ void debug_handler(int sig, siginfo_t *si, void *arg)
- }
-
- /* now wait for all others to finish dumping. The last one will set all
- - * bits again to broadcast the leaving condition.
- + * bits again to broadcast the leaving condition so we'll see ourselves
- + * present again. This way the threads_to_dump variable never passes to
- + * zero until all visitors have stopped waiting.
- */
- - while (threads_to_dump & all_threads_mask) {
- - if (threads_to_dump & tid_bit)
- - HA_ATOMIC_AND(&threads_to_dump, ~tid_bit);
- - else
- - ha_thread_relax();
- - }
- + while (!(threads_to_dump & tid_bit))
- + ha_thread_relax();
- + HA_ATOMIC_AND(&threads_to_dump, ~tid_bit);
-
- /* mark the current thread as stuck to detect it upon next invocation
- * if it didn't move.
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