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SubjectRe: next-20081215: kernel BUG at block/cfq-iosched.c:1243
On Mon, Dec 15 2008, Alexander Beregalov wrote:
> 2008/12/15 Jens Axboe <jens.axboe@oracle.com>:
> >> [ 23.080733] kernel BUG at block/cfq-iosched.c:1243!
> >> [ 23.080733] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>
> > Odd, is it reproducible? What is your .config?
> Yes, it is
>
> Config is in attach

It's weird, because the only change for CFQ in that area is the one
below. Does the oops go away if you back this out?


From 030a3f02bb42fad91ae84c99924fe3ee162a634d Mon Sep 17 00:00:00 2001
From: Jens Axboe <jens.axboe@oracle.com>
Date: Fri, 12 Dec 2008 15:56:31 +0100
Subject: [PATCH] cfq-iosched: fix race between exiting queue and exiting task

Original patch from Nikanth Karthikesan <knikanth@suse.de>

When a queue exits the queue lock is taken and cfq_exit_queue() would free all
the cic's associated with the queue.

But when a task exits, cfq_exit_io_context() gets cic one by one and then
locks the associated queue to call __cfq_exit_single_io_context. It looks like
between getting a cic from the ioc and locking the queue, the queue might have
exited on another cpu.

Fix this by rechecking the cfq_io_context queue key inside the queue lock
again, and not calling into __cfq_exit_single_io_context() if somebody
beat us to it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
block/cfq-iosched.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ee8a90c..8a2485d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1314,7 +1314,16 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
unsigned long flags;

spin_lock_irqsave(q->queue_lock, flags);
+
+ /*
+ * Ensure we get a fresh copy of the ->key to prevent
+ * race between exiting task and queue
+ */
+ smp_read_barrier_depends();
+ if (cic->key)
+ __cfq_exit_single_io_context(cfqd, cic);
__cfq_exit_single_io_context(cfqd, cic);
+
spin_unlock_irqrestore(q->queue_lock, flags);
}
}
--
1.6.0.4.755.g6e139

--
Jens Axboe



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