Re: [PATCH] scsi: core: cleanup request queue before releasing gendisk

Ming Lei
2021-09-17T08:32:14Z

On Fri, Sep 17, 2021 at 09:56:50AM +0200, Christoph Hellwig wrote:
> On Fri, Sep 17, 2021 at 03:41:05PM +0800, Ming Lei wrote:
> > >
> > > - return ret;
> > > + if (unlikely(!disk_live(disk))) {
> > > + blk_queue_exit(disk->queue);
> > > + bio_io_error(bio);
> > > + return -ENODEV;
> > > + }
> >
> > Is it safe to fail IO in this way? There is still opened bdev, and
> > usually IO can be done successfully even when disk is being deleted.
>
> "normal" I/O should not really happen by the time it is deleted. That
> being said we should do this only after the fsync is done. While no
> one should rely on that I'm pretty sure some file systems do.
> So we'll actually need a deleted flag.
>
> > Not mention it adds one extra check in real fast path.
>
> I'm not really woried about the check itself. That being
> sais this inode cache line is not hot right now, so moving it to
> disk->state will help as we need to check the read-only flag in
> the the I/O submission path anyway.

When the deleted flag is set in del_gendisk(), it may not be observed
in time in bio_submit() because of memory/compiler re-order, so the
check is still sort of relax constraint. I guess the same is true for
read-only check.

>
> > > + /*
> > > + * Prevent new I/O from crossing bio_queue_enter().
> > > + */
> > > + blk_freeze_queue_start(q);
> > > + if (queue_is_mq(q))
> > > + blk_mq_wake_waiters(q);
> > > + /* Make blk_queue_enter() reexamine the DYING flag. */
> > > + wake_up_all(&q->mq_freeze_wq);
> > > +
> > > + rq_qos_exit(q);
> >
> > rq_qos_exit() requires the queue to be frozen, otherwise kernel oops
> > can be triggered. There may be old submitted bios not completed yet,
> > and rq_qos_done() can be referring to q->rq_qos.
>
> Yes. I actually misread the old code - it atually does two
> blk_freeze_queue_start, but it also includes the wait.

Only blk_freeze_queue() includes the wait, and blk_freeze_queue_start()
doesn't.

>
> > But if waiting for queue frozen is added, one extra freeze is added,
> > which will slow done remove disk/queue.
>
> Is it? For the typical case the second free in blk_cleanp_queue will
> be bsically free because there is no pending I/O. The only case
> where that matters if if there is pending passthrough I/O again,
> which can only happen with SCSI, and even there is highly unusual.

RCU grace period is involved in blk_freeze_queue(). One way you can
avoid it is to keep the percpu ref at atomic mode when running
blk_mq_unfreeze_queue() in del_gendisk().

Thanks,
Ming