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>

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index b8d4182d14ccb47c2b3d1014b8976cb58614434d..8cf0678378d2976ddb0da2ade0bd216fc8689eac 100644 (file)
--- 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);