BUG/MEDIUM: time: fix updating of global_now upon clock drift

During commit 7e4a557f6 ("MINOR: time: change the global timeval and the
the global tick at once") the approach made sure that the new now_ms was
always higher than or equal to global_now_ms, but by forgetting the old
value. This can cause the first update to global_now_ms to fail if it's
already out of sync, going back into the loop, and the subsequent call
would then succeed due to commit 4d01f3dcd ("MINOR: time: avoid
overwriting the same values of global_now").

And if it goes out of sync, it will fail to update forever, as observed
by Ashley Penney in github issue #1194, causing incorrect freq counters
calculations everywhere. One possible trigger for this issue is one thread
spinning for a few milliseconds while the other ones continue to work.

The issue really is that old_now_ms ought not to be modified in the loop
as it's used for the CAS. But we don't need to structurally guarantee that
global_now_ms grows monotonically as it's computed from the new global_now
which is already verified for this via the __tv_islt() test. Thus, dropping
any corrections on global_now_ms in the loop is the correct way to proceed
as long as this one is always updated to follow global_now.

No backport is needed, this is only for 2.4-dev.

diff --git a/src/time.c b/src/time.c
index 4ebde3779fc786552d18f01180945da88d7eeafe..d6ab185b41163ff390b2fb6ded18c828d50eaf16 100644 (file)
--- a/src/time.c
+++ b/src/time.c
@@ -236,7 +236,6 @@ void tv_update_date(int max_wait, int interrupted)
        do {
                tmp_now.tv_sec  = (unsigned int)(old_now >> 32);
                tmp_now.tv_usec = old_now & 0xFFFFFFFFU;
-               old_now_ms = __tv_to_ms(&tmp_now);
 
                if (__tv_islt(&now, &tmp_now))
                        now = tmp_now;
@@ -246,8 +245,6 @@ void tv_update_date(int max_wait, int interrupted)
                 */
                new_now = ((ullong)now.tv_sec << 32) + (uint)now.tv_usec;
                now_ms = __tv_to_ms(&now);
-               if (tick_is_lt(now_ms, old_now_ms))
-                       now_ms = old_now_ms;
 
                /* let's try to update the global <now> (both in timeval
                 * and ms forms) or loop again.