linux-kernel - Re: [PATCH] fix? current_io_context() vs set_task

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Date:	Mon, 21 Aug 2006 08:28:54 +0200
From:	Jens Axboe <axboe@...e.de>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix? current_io_context() vs set_task_ioprio() race

On Sun, Aug 20 2006, Oleg Nesterov wrote:
> I know nothing about io scheduler, but I suspect set_task_ioprio() is not safe.
> 
> current_io_context() initializes "struct io_context", then sets ->io_context.
> set_task_ioprio() running on another cpu may see the changes out of order, so
> ->set_ioprio(ioc) may use io_context which was not initialized properly.
> 
> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> 
> --- 2.6.18-rc4/block/ll_rw_blk.c~3_race	2006-07-16 01:53:08.000000000 +0400
> +++ 2.6.18-rc4/block/ll_rw_blk.c	2006-08-20 19:30:10.000000000 +0400
> @@ -3628,6 +3628,8 @@ struct io_context *current_io_context(gf
>  		ret->nr_batch_requests = 0; /* because this is 0 */
>  		ret->aic = NULL;
>  		ret->cic_root.rb_node = NULL;
> +		/* make sure set_task_ioprio() sees the settings above */
> +		smp_wmb();
>  		tsk->io_context = ret;
>  	}
>  
> --- 2.6.18-rc4/fs/ioprio.c~3_race	2006-08-20 19:09:08.000000000 +0400
> +++ 2.6.18-rc4/fs/ioprio.c	2006-08-20 19:57:14.000000000 +0400
> @@ -44,6 +44,9 @@ static int set_task_ioprio(struct task_s
>  	task->ioprio = ioprio;
>  
>  	ioc = task->io_context;
> +	/* see wmb() in current_io_context() */
> +	smp_read_barrier_depends();
> +
>  	if (ioc && ioc->set_ioprio)
>  		ioc->set_ioprio(ioc, ioprio);

Agree, applied.

-- 
Jens Axboe

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