[PATCH AUTOSEL 5.12 115/116] watchdog: fix barriers when printing backtraces from all CPUs

From: Sasha Levin
Date: Wed May 05 2021 - 12:42:53 EST


From: Petr Mladek <pmladek@xxxxxxxx>

[ Upstream commit 9f113bf760ca90d709f8f89a733d10abb1f04a83 ]

Any parallel softlockup reports are skipped when one CPU is already
printing backtraces from all CPUs.

The exclusive rights are synchronized using one bit in
soft_lockup_nmi_warn. There is also one memory barrier that does not make
much sense.

Use two barriers on the right location to prevent mixing two reports.

[pmladek@xxxxxxxx: use bit lock operations to prevent multiple soft-lockup reports]
Link: https://lkml.kernel.org/r/YFSVsLGVWMXTvlbk@alley

Link: https://lkml.kernel.org/r/20210311122130.6788-6-pmladek@xxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Laurence Oberman <loberman@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
kernel/watchdog.c | 17 ++++++-----------
1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index b8d4182d14cc..8cf0678378d2 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -410,11 +410,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
if (kvm_check_and_clear_guest_paused())
return HRTIMER_RESTART;

+ /*
+ * Prevent multiple soft-lockup reports if one cpu is already
+ * engaged in dumping all cpu back traces.
+ */
if (softlockup_all_cpu_backtrace) {
- /* Prevent multiple soft-lockup reports if one cpu is already
- * engaged in dumping cpu back traces
- */
- if (test_and_set_bit(0, &soft_lockup_nmi_warn))
+ if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn))
return HRTIMER_RESTART;
}

@@ -432,14 +433,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
dump_stack();

if (softlockup_all_cpu_backtrace) {
- /* Avoid generating two back traces for current
- * given that one is already made above
- */
trigger_allbutself_cpu_backtrace();
-
- clear_bit(0, &soft_lockup_nmi_warn);
- /* Barrier to sync with other cpus */
- smp_mb__after_atomic();
+ clear_bit_unlock(0, &soft_lockup_nmi_warn);
}

add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
--
2.30.2