linux-kernel - [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug code

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Message-Id: <1296496389-30647-1-git-send-email-will.deacon@arm.com>
Date:	Mon, 31 Jan 2011 17:53:09 +0000
From:	Will Deacon <will.deacon@....com>
To:	linux-kernel@...r.kernel.org, oprofile-list@...ts.sf.net
Cc:	Will Deacon <will.deacon@....com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Robert Richter <robert.richter@....com>
Subject: [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug code

OProfile uses a CPU notifier to start and stop any hrtimers when CPUs change
between ONLINE and DEAD. A static int ctr_running is used to keep track of
the counter state.

This can lead to problems where writes to the state variable are re-ordered
with repect to reads of the variable occurring on other CPUs, meaning that
__oprofile_hrtimer_start may read ctr_running as 0 and not initialise the
hrtimer. Potential deadlock can occur in __oprofile_hrtimer_stop because
lock_hrtimer_base will poll until timer->base != NULL, which will never
happen.

This patch adds an smp_mb() before initialising the hrtimers to ensure that
ctr_running mirrors the correct counter state.

Cc: Santosh Shilimkar <santosh.shilimkar@...com>
Cc: Robert Richter <robert.richter@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
---

Changes since v1: Removed redundant barrier from stop() code.

 drivers/oprofile/timer_int.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/drivers/oprofile/timer_int.c b/drivers/oprofile/timer_int.c
index 0107251..a3a94ea 100644
--- a/drivers/oprofile/timer_int.c
+++ b/drivers/oprofile/timer_int.c
@@ -48,6 +48,7 @@ static int oprofile_hrtimer_start(void)
 {
 	get_online_cpus();
 	ctr_running = 1;
+	smp_mb();
 	on_each_cpu(__oprofile_hrtimer_start, NULL, 1);
 	put_online_cpus();
 	return 0;
-- 
1.7.0.4

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