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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Laurence Oberman <loberman@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincent Whitchurch <vincent.whitchurch@axis.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.12 122/363] watchdog: fix barriers when printing backtraces from all CPUs
Date:   Mon, 17 May 2021 15:59:48 +0200
Message-Id: <20210517140306.751014454@linuxfoundation.org>
In-Reply-To: <20210517140302.508966430@linuxfoundation.org>
References: <20210517140302.508966430@linuxfoundation.org>
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From: Petr Mladek <pmladek@suse.com>

[ 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@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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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