[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 <redacted>
Signed-off-by: Stephen Boyd <redacted>
---

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

-- 
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
hosted by The Linux Foundation

On 09/05/12 14:01, Stephen Boyd wrote:

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 <redacted>
Signed-off-by: Stephen Boyd <redacted>
---

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

Any comments?

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation