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Date:   Tue, 15 May 2018 13:48:55 -0700
From:   tip-bot for Dexuan Cui <tipbot@zytor.com>
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:timers/urgent] tick/broadcast: Use for_each_cpu() specially on
 UP kernels
Git-Commit-ID: 5596fe34495cf0f645f417eb928ef224df3e3cb4
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Commit-ID:  5596fe34495cf0f645f417eb928ef224df3e3cb4
Gitweb:     https://git.kernel.org/tip/5596fe34495cf0f645f417eb928ef224df3e3cb4
Author:     Dexuan Cui <decui@microsoft.com>
AuthorDate: Tue, 15 May 2018 19:52:50 +0000
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 15 May 2018 22:45:54 +0200

tick/broadcast: Use for_each_cpu() specially on UP kernels

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 1~20
minutes during boot-up, and sometimes it can hang forever.

Protect if by checking whether the cpumask is empty before entering the
for_each_cpu() loop.

[ tglx: Use !IS_ENABLED(CONFIG_SMP) instead of #ifdeffery ]

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poulson <jopoulso@microsoft.com>
Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org
Cc: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Jork Loeser <Jork.Loeser@microsoft.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: KY Srinivasan <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Link: https://lkml.kernel.org/r/KL1P15301MB000678289FE55BA365B3279ABF990@KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM
Link: https://lkml.kernel.org/r/KL1P15301MB0006FA63BC22BEB64902EAA0BF930@KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM
---
 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 b398c2ea69b2..aa2094d5dd27 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 = ktime_get();
 	/* Find all expired events */
 	for_each_cpu(cpu, tick_broadcast_oneshot_mask) {
+		/*
+		 * Required for !SMP because for_each_cpu() reports
+		 * unconditionally CPU0 as set on UP kernels.
+		 */
+		if (!IS_ENABLED(CONFIG_SMP) &&
+		    cpumask_empty(tick_broadcast_oneshot_mask))
+			break;
+
 		td = &per_cpu(tick_cpu_device, cpu);
 		if (td->evtdev->next_event <= now) {
 			cpumask_set_cpu(cpu, tmpmask);