linux-kernel - Re: [patch 1/2]scsi: scsi_run_queue() doesn't use local list to handle starved sdev

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Date:	Thu, 22 Dec 2011 18:27:29 +0000
From:	James Bottomley <jbottomley@...allels.com>
To:	Shaohua Li <shaohua.li@...el.com>
CC:	lkml <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Ted Ts'o <tytso@....edu>,
	"Wu, Fengguang" <fengguang.wu@...el.com>,
	"Darrick J. Wong" <djwong@...ibm.com>
Subject: Re: [patch 1/2]scsi: scsi_run_queue() doesn't use local list to
 handle starved sdev

On Thu, 2011-12-22 at 11:10 +0800, Shaohua Li wrote:
> scsi_run_queue() picks off all sdev from host starved_list to a local list,
> then handle them. If there are multiple threads running scsi_run_queue(),
> the starved_list will get messed. This is quite common, because request
> rq_affinity is on by default.
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> ---
>  drivers/scsi/scsi_lib.c |   21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> Index: linux/drivers/scsi/scsi_lib.c
> ===================================================================
> --- linux.orig/drivers/scsi/scsi_lib.c	2011-12-21 16:56:23.000000000 +0800
> +++ linux/drivers/scsi/scsi_lib.c	2011-12-22 09:33:09.000000000 +0800
> @@ -401,9 +401,8 @@ static inline int scsi_host_is_busy(stru
>   */
>  static void scsi_run_queue(struct request_queue *q)
>  {
> -	struct scsi_device *sdev = q->queuedata;
> +	struct scsi_device *sdev = q->queuedata, *head_sdev = NULL;
>  	struct Scsi_Host *shost;
> -	LIST_HEAD(starved_list);
>  	unsigned long flags;
>  
>  	/* if the device is dead, sdev will be NULL, so no queue to run */
> @@ -415,9 +414,8 @@ static void scsi_run_queue(struct reques
>  		scsi_single_lun_run(sdev);
>  
>  	spin_lock_irqsave(shost->host_lock, flags);
> -	list_splice_init(&shost->starved_list, &starved_list);
>  
> -	while (!list_empty(&starved_list)) {
> +	while (!list_empty(&shost->starved_list)) {

The original reason for working from a copy instead of the original list
was that the device can end up back on the starved list because of a
variety of conditions in the HBA and so this would cause the loop not to
exit, so this piece of the patch doesn't look right to me.

James