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watchdog: fix barriers when printing backtraces from all CPUs 

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@suse.com: 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@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Petr Mladek 2021-04-29 22:54:33 -07:00 committed by Linus Torvalds
parent 1bc503cb4a
commit 9f113bf760
1 changed files with 6 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
kernel/watchdog.c
View File

@ -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);