[PATCH 24/72] panic: fix a possible deadlock in panic()

[Luis Henriques]

3.5.7.11 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vikram Mulukutla <markivx at codeaurora.org>

commit 190320c3b6640d4104650f55ff69611e050ea06b upstream.

panic_lock is meant to ensure that panic processing takes place only on
one cpu; if any of the other cpus encounter a panic, they will spin
waiting to be shut down.

However, this causes a regression in this scenario:

1. Cpu 0 encounters a panic and acquires the panic_lock
   and proceeds with the panic processing.
2. There is an interrupt on cpu 0 that also encounters
   an error condition and invokes panic.
3. This second invocation fails to acquire the panic_lock
   and enters the infinite while loop in panic_smp_self_stop.

Thus all panic processing is stopped, and the cpu is stuck for eternity
in the while(1) inside panic_smp_self_stop.

To address this, disable local interrupts with local_irq_disable before
acquiring the panic_lock.  This will prevent interrupt handlers from
executing during the panic processing, thus avoiding this particular
problem.

Signed-off-by: Vikram Mulukutla <markivx at codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd at codeaurora.org>
Cc: Michael Holzheu <holzheu at linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/panic.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/panic.c b/kernel/panic.c
index d2a5f4e..e1b2822 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -75,6 +75,14 @@ void panic(const char *fmt, ...)
 	int state = 0;
 
 	/*
+	 * Disable local interrupts. This will prevent panic_smp_self_stop
+	 * from deadlocking the first cpu that invokes the panic, since
+	 * there is nothing to prevent an interrupt handler (that runs
+	 * after the panic_lock is acquired) from invoking panic again.
+	 */
+	local_irq_disable();
+
+	/*
 	 * It's possible to come here directly from a panic-assertion and
 	 * not have preempt disabled. Some functions called from here want
 	 * preempt to be disabled. No point enabling it later though...
-- 
1.8.1.2