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.
diff --git a/src/debug.c b/src/debug.c
index 6b9d149..9f538b6 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -439,8 +439,8 @@
* 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 */
@@ -453,7 +453,7 @@
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
@@ -461,14 +461,13 @@
}
/* 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.