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From: Aaron Carroll <aaronc@gelato.unsw.edu.au>
To: "Alan D. Brunelle" <Alan.Brunelle@hp.com>
Date: Thu, 24 Apr 2008 10:37:28 +1000
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CC: linux-kernel@vger.kernel.org, Jens Axboe <jens.axboe@oracle.com>
Subject: Re: [RFC][PATCH 3/3] Do not use rqhash when merges disabled
References: <480F8936.5030406@hp.com> <480F8AB9.2050805@hp.com>
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Hi Alan,

Alan D. Brunelle wrote:
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -313,7 +313,7 @@ static inline void __elv_rqhash_del(struct request *rq)
>  
>  static void elv_rqhash_del(struct request_queue *q, struct request *rq)
>  {
> -	if (ELV_ON_HASH(rq))
> +	if (!blk_queue_nomerges(q) && ELV_ON_HASH(rq))
>  		__elv_rqhash_del(rq);
>  }

If you switch the nomerges tunable while requests are in flight, it is possible that
a request is put into the rqhash table but not removed here, leading to the BUG_ON
in elv_dequeue_request() triggering.  ELV_ON_HASH needs to be checked regardless of
the nomerges state.


 -- Aaron