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vlib: Handle race in thread barrier processing

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When CLIB_DEBUG is enabled, vlib_foreach_main macro asserts that
vlib_main it currently looks at is safely parked in barrier, by
checkling that vlib_main->parked_at_barrier is not 0.
Unfortunately, the check is racy - workers first increment the
atomic counter to indicate that they have reached the barrier
and _then_ set this_main->parked_at_barrier to 1. For the last
worker to suspend this opens the race - main thread is free
to execute and assert immediately after atomic counter has been
incremented, before worker gets to write to own parked_at_barrier.

Fix this by simply swapping the order of two operations.

Type: fix

Signed-off-by: Alexnader Kabaev <kan@FreeBSD.org>
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: Iae47abd6ca0be1c5413f5ecaefabc64cd7eac2ed
(cherry picked from commit feda545105)
This commit is contained in:
Alexander Kabaev
2019-11-01 18:26:20 -04:00
committed by Andrew Yourtchenko
parent f821dd7990
commit ccdd73e636
1 changed files with 1 additions and 1 deletions
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2
src/vlib/threads.h
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@ -416,12 +416,12 @@ vlib_worker_thread_barrier_check (void)
ed->thread_index = thread_index; ed->thread_index = thread_index;
} }
clib_atomic_fetch_add (vlib_worker_threads->workers_at_barrier, 1);
if (CLIB_DEBUG > 0) if (CLIB_DEBUG > 0)
{ {
vm = vlib_get_main (); vm = vlib_get_main ();
vm->parked_at_barrier = 1; vm->parked_at_barrier = 1;
} }
clib_atomic_fetch_add (vlib_worker_threads->workers_at_barrier, 1);
while (*vlib_worker_threads->wait_at_barrier) while (*vlib_worker_threads->wait_at_barrier)
; ;