linux-kernel - [PATCH] ARM: Fix deadlock scenario with smp_send

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Message-Id: <1346878886-7159-1-git-send-email-sboyd@codeaurora.org>
Date:	Wed,  5 Sep 2012 14:01:26 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: [PATCH] ARM: Fix deadlock scenario with smp_send_stop()

If one process calls sys_reboot and that process then stops other
CPUs while those CPUs are within a spin_lock() region we can
potentially encounter a deadlock scenario like below.

CPU 0                   CPU 1
-----                   -----
                        spin_lock(my_lock)
smp_send_stop()
 send_IPI               handle_IPI()
                         disable_preemption/irqs
                          while(1);
 <PREEMPT>
spin_lock(my_lock) <--- Waits forever

We shouldn't attempt to run any other tasks after we send a stop
IPI to a CPU so disable preemption so that the reboot task runs to
completion.

Reported-by: Sundarajan Srinivasan <sundaraj@...eaurora.com>
Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
---

Unfortunately this won't fix the other problem I mentioned two weeks
ago where smp_send_stop races with other CPUs calling cpu_up().

 arch/arm/kernel/process.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 693b744..663a7a8 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -239,6 +239,7 @@ __setup("reboot=", reboot_setup);
 
 void machine_shutdown(void)
 {
+	preempt_disable();
 #ifdef CONFIG_SMP
 	smp_send_stop();
 #endif
-- 
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