commit 7607ec0917c33ce511d46b791dfa5550451dd538
Author: Willy Tarreau <w@1wt.eu>
Date:   Tue Aug 7 10:07:15 2018 +0200

    BUG/MEDIUM: threads: fix the no-thread case after the change to the sync point
    
    In commit 0c026f4 ("MINOR: threads: add more consistency between certain
    variables in no-thread case"), we ensured that we don't have all_threads_mask
    zeroed anymore. But one test was missed for the write() to the sync pipe.
    This results in a situation where when running single-threaded, once a
    server status changes, a wake-up message is written to the pipe and never
    consumed, showing a 100% CPU usage.
    
    No backport is needed.
    (cherry picked from commit ab657ce2511c4e19b0191fbe1c98cfd823a3c5d6)
    [wt: the offending patch was just backported as the previous one]
    Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/src/hathreads.c b/src/hathreads.c
index 238cbb80..ba05fe27 100644
--- a/src/hathreads.c
+++ b/src/hathreads.c
@@ -71,7 +71,7 @@ void thread_sync_enable(void)
  */
 void thread_want_sync()
 {
-	if (all_threads_mask) {
+	if (all_threads_mask & (all_threads_mask - 1)) {
 		if (threads_want_sync & tid_bit)
 			return;
 		if (HA_ATOMIC_OR(&threads_want_sync, tid_bit) == tid_bit)