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From:   Dexuan Cui <decui@microsoft.com>
To:     Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: [PATCH] tick/broadcast: Use for_each_cpu() specially on UP kernels
Thread-Topic: [PATCH] tick/broadcast: Use for_each_cpu() specially on UP
 kernels
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Date:   Tue, 15 May 2018 19:52:50 +0000
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for_each_cpu() unintuitively reports CPU0 as set independent of the actual
cpumask content on UP kernels. This causes an unexpected PIT interrupt
storm on a UP kernel running in an SMP virtual machine on Hyper-V, and as
a result, the virtual machine can suffer from a strange random delay of 1~2=
0
minutes during boot-up, and sometimes it can hang forever.

Link: https://lkml.org/lkml/2018/5/9/63
Link: https://lkml.org/lkml/2018/5/15/747
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org
---

Some part of the changelog and comments are copied from=20
Thomas Gleixner's 115ef3b7e61a ("watchdog/hardlockup/perf: Cure UP damage")=
 :-)

 kernel/time/tick-broadcast.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index b398c2e..2fc59ad 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -612,6 +612,14 @@ static void tick_handle_oneshot_broadcast(struct clock=
_event_device *dev)
 	now =3D ktime_get();
 	/* Find all expired events */
 	for_each_cpu(cpu, tick_broadcast_oneshot_mask) {
+#ifndef CONFIG_SMP
+		/*
+		 * Required because for_each_cpu() reports unconditionally
+		 * CPU0 as set on UP kernels.
+		 */
+		if (cpumask_empty(tick_broadcast_oneshot_mask))
+			break;
+#endif
 		td =3D &per_cpu(tick_cpu_device, cpu);
 		if (td->evtdev->next_event <=3D now) {
 			cpumask_set_cpu(cpu, tmpmask);
--=20
2.7.4