linux-kernel - [PATCH 6/9] blackfin: Fix possible deadlock in decode

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Message-ID: <20120423070915.GF30752@lizard>
Date:	Mon, 23 Apr 2012 00:09:15 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>
Cc:	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Weinberger <richard@....at>,
	Paul Mundt <lethal@...ux-sh.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	John Stultz <john.stultz@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	linux-mm@...ck.org
Subject: [PATCH 6/9] blackfin: Fix possible deadlock in decode_address()

Oleg Nesterov found an interesting deadlock possibility:

> sysrq_showregs_othercpus() does smp_call_function(showacpu)
> and showacpu() show_stack()->decode_address(). Now suppose that IPI
> interrupts the task holding read_lock(tasklist).

To fix this, blackfin should not grab the write_ variant of the
tasklist lock, read_ one is enough.

Suggested-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@...aro.org>
---
 arch/blackfin/kernel/trace.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index d08f0e3..f7f7a18 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -29,7 +29,7 @@ void decode_address(char *buf, unsigned long address)
 {
 	struct task_struct *p;
 	struct mm_struct *mm;
-	unsigned long flags, offset;
+	unsigned long offset;
 	struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -113,7 +113,7 @@ void decode_address(char *buf, unsigned long address)
 	 * mappings of all our processes and see if we can't be a whee
 	 * bit more specific
 	 */
-	write_lock_irqsave(&tasklist_lock, flags);
+	read_lock(&tasklist_lock);
 	for_each_process(p) {
 		struct task_struct *t;
 
@@ -186,7 +186,7 @@ __continue:
 	sprintf(buf, "/* kernel dynamic memory */");
 
 done:
-	write_unlock_irqrestore(&tasklist_lock, flags);
+	read_unlock(&tasklist_lock);
 }
 
 #define EXPAND_LEN ((1 << CONFIG_DEBUG_BFIN_HWTRACE_EXPAND_LEN) * 256 - 1)
-- 
1.7.9.2

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