linux-kernel - Re: [PATCH] Exiting queue and task might race to free cic

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Date:	Wed, 19 Nov 2008 15:15:31 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	linux-kernel@...r.kernel.org, Fabio Checconi <fchecconi@...il.com>
Subject: Re: [PATCH] Exiting queue and task might race to free cic

On Wed, Nov 19 2008, Nikanth Karthikesan wrote:
> Hi Jens
> 
> Looking at the bug reported here
> http://thread.gmane.org/gmane.linux.kernel/722539
> it looks like an exiting queue can race with an exiting task.
> 
> 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. Isn't this possible?
> 
> If possible, either verifying whether cic->key is still not null or q->flags 
> does not have QUEUE_FLAG_DEAD set would fix this.
> 
> Thanks
> Nikanth Karthikesan
> 
> Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>
> 
> ---
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 6a062ee..b9b627a 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1318,7 +1318,12 @@ static void cfq_exit_single_io_context(struct 
> io_context *ioc,
>  		unsigned long flags;
>  
>  		spin_lock_irqsave(q->queue_lock, flags);
> -		__cfq_exit_single_io_context(cfqd, cic);
> +		/*
> +		 * cic might have been already exited when an exiting task
> +		 * races with an exiting queue.
> +		 */
> +		if (likely(cic->key))
> +			__cfq_exit_single_io_context(cfqd, cic);
>  		spin_unlock_irqrestore(q->queue_lock, flags);
>  	}
>  }

Not sure this is enough, we probably need to copy the key to ensure that
we get a fresh value. How does this look?

Did you actually trigger this, or is it just from code inspection?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..560cd1c 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1318,7 +1318,14 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
 		unsigned long flags;
 
 		spin_lock_irqsave(q->queue_lock, flags);
-		__cfq_exit_single_io_context(cfqd, cic);
+
+		/*
+		 * 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);
 		spin_unlock_irqrestore(q->queue_lock, flags);
 	}
 }

-- 
Jens Axboe

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