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SubjectRe: Strange block/scsi/workqueue issue
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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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