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Date:	Thu, 20 Dec 2012 15:12:48 +0000
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Luck, Tony (tony.luck@...el.com)" <tony.luck@...el.com>,
	"dzickus@...hat.com" <dzickus@...hat.com>
CC:	"ccross@...roid.com" <ccross@...roid.com>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"cbouatmailru@...il.com" <cbouatmailru@...il.com>,
	Satoru Moriya <satoru.moriya@....com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>
Subject: [PATCH v3 2/2] efi_pstore: Avoid deadlock in non-blocking paths

[Issue]

There is a scenario which efi_pstore may hang up:

 - cpuA grabs efivars->lock
 - cpuB panics and calls smp_send_stop
 - smp_send_stop sends IRQ to cpuA
 - after 1 second, cpuB gives up on cpuA and sends an NMI instead
 - cpuA is now in an NMI handler while still holding efivars->lock
 - cpuB is deadlocked

This case may happen if a firmware has a bug and
cpuA is stuck talking with it.

[Solution]

This patch changes a spin_lock to a spin_trylock in non-blocking paths.
and if the spin_lock has already taken by another cpu,
it returns without accessing to a firmware to avoid the deadlock.

Signed-off-by: Seiji Aguchi <seiji.aguchi@....com>
---
 drivers/firmware/efivars.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index 7b1c374..ef5070d 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1209,7 +1209,16 @@ static int efi_pstore_write(enum pstore_type_id type,
 	u64 storage_space, remaining_space, max_variable_size;
 	efi_status_t status = EFI_NOT_FOUND;
 
-	spin_lock(&efivars->lock);
+	if (pstore_cannot_block_path(reason)) {
+		/*
+		 * If the lock is taken by another cpu in non-blocking path,
+		 * this driver returns without entering firmware to avoid
+		 * hanging up.
+		 */
+		if (!spin_trylock(&efivars->lock))
+			return -EBUSY;
+	} else
+		spin_lock(&efivars->lock);
 
 	/*
 	 * Check if there is a space enough to log.
-- 1.7.1
--
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