linux-kernel - Re: Strange block/scsi/workqueue issue

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Message-ID: <1302725866.2597.33.camel@mulgrave.site>
Date:	Wed, 13 Apr 2011 15:17:46 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Steven Whitehouse <swhiteho@...hat.com>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Strange block/scsi/workqueue issue

On Wed, 2011-04-13 at 22:12 +0200, Jens Axboe wrote:
> On 2011-04-13 19:01, James Bottomley wrote:
> > While you still have the problematic system, can you try this patch?  It
> > avoids changing anything in block (other than to add a missing state
> > guard for the elv_next_request).  If it works, we can defer the sync vs
> > async discussion and use it for a -stable fix.
> > 
> > Thanks,
> > 
> > James
> > 
> > ---
> > 
> > diff --git a/block/blk.h b/block/blk.h
> > index c8db371..11d0d25 100644
> > --- a/block/blk.h
> > +++ b/block/blk.h
> > @@ -62,7 +62,8 @@ static inline struct request *__elv_next_request(struct request_queue *q)
> >  			return rq;
> >  		}
> >  
> > -		if (!q->elevator->ops->elevator_dispatch_fn(q, 0))
> > +		if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags) ||
> > +		    !q->elevator->ops->elevator_dispatch_fn(q, 0))
> >  			return NULL;
> >  	}
> >  }
> > diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
> > index e44ff64..5aa4246 100644
> > --- a/drivers/scsi/scsi_sysfs.c
> > +++ b/drivers/scsi/scsi_sysfs.c
> > @@ -322,14 +322,9 @@ static void scsi_device_dev_release_usercontext(struct work_struct *work)
> >  		kfree(evt);
> >  	}
> >  
> > -	if (sdev->request_queue) {
> > -		sdev->request_queue->queuedata = NULL;
> > -		/* user context needed to free queue */
> > -		scsi_free_queue(sdev->request_queue);
> > -		/* temporary expedient, try to catch use of queue lock
> > -		 * after free of sdev */
> > -		sdev->request_queue = NULL;
> > -	}
> > +	/* temporary expedient, try to catch use of queue lock after
> > +	 * free of sdev */
> > +	sdev->request_queue = NULL;
> >  
> >  	scsi_target_reap(scsi_target(sdev));
> >  
> > @@ -937,6 +932,11 @@ void __scsi_remove_device(struct scsi_device *sdev)
> >  	if (sdev->host->hostt->slave_destroy)
> >  		sdev->host->hostt->slave_destroy(sdev);
> >  	transport_destroy_device(dev);
> > +	/* Setting this to NULL causes the request function to reject
> > +	 * any I/O requests */
> > +	sdev->request_queue->queuedata = NULL;
> > +	/* Freeing the queue signals to block that we're done */
> > +	scsi_free_queue(sdev->request_queue);
> >  	put_device(dev);
> >  }
> 
> This patch looks pretty clean. Shouldn't you serialize that ->queuedata
> = NULL assignment with the queue lock, though?

pointer assignment is atomic, isn't it?  As in on a 32 bit arch, it
definitely is, and I thought on 64 bits it also was.  So a simultaneous
reader either sees previous value or NULL.

James



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