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

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Date:	Fri, 23 Dec 2011 08:40:51 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	James Bottomley <jbottomley@...allels.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 18:27 +0000, James Bottomley wrote:
> 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.
+               /*
+                * the head sdev is no longer starved and removed from the
+                * starved list, select a new sdev as head.
+                */
+               if (head_sdev == sdev && list_empty(&sdev->starved_entry))
+                       head_sdev = NULL;
I had this in the loop, which is to guarantee the loop will exit if a
device is removed from the starved list.

Thanks,
Shaohua

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