On Fri, 2015-04-17 at 15:44 -0600, Jens Axboe wrote:
> On 04/17/2015 03:42 PM, James Bottomley wrote:
> >> @@ -662,32 +662,14 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
> >> */
> >> int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
> >> {
> >> - unsigned long flags;
> >> -
> >> - if (depth <= 0)
> >> - goto out;
> >> -
> >> - spin_lock_irqsave(sdev->request_queue->queue_lock, flags);
> >> + if (depth > 0) {
> >> + unsigned long flags;
> >>
> >> - /*
> >> - * Check to see if the queue is managed by the block layer.
> >> - * If it is, and we fail to adjust the depth, exit.
> >> - *
> >> - * Do not resize the tag map if it is a host wide share bqt,
> >> - * because the size should be the hosts's can_queue. If there
> >> - * is more IO than the LLD's can_queue (so there are not enuogh
> >> - * tags) request_fn's host queue ready check will handle it.
> >> - */
> >> - if (!shost_use_blk_mq(sdev->host) && !sdev->host->bqt) {
> >> - if (blk_queue_tagged(sdev->request_queue) &&
> >> - blk_queue_resize_tags(sdev->request_queue, depth) != 0)
> >> - goto out_unlock;
> >> + spin_lock_irqsave(sdev->request_queue->queue_lock, flags);
> >> + sdev->queue_depth = depth;
> >> + spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
> >
> > This lock/unlock is a nasty global sync point which can be eliminated:
> > we can rely on the architectural atomicity of 32 bit writes (might need
> > to make sdev->queue_depth a u32 because I seem to remember 16 bit writes
> > had to be done as two byte stores on some architectures).
>
> It's not in a hot path (by any stretch), so doesn't really matter...
Sure, but it's good practise not to do this, otherwise the pattern
lock/u32 store/unlock gets duplicated into hot paths by people who are
confused about whether locking is required.
James
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