linux-kernel - Re: [PATCH] block, loop: Increment diskseq when releasing a loop device

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Date:   Wed, 7 Jun 2023 12:42:00 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block, loop: Increment diskseq when releasing a loop
 device

On Wed, Jun 07, 2023 at 11:23:00AM -0400, Demi Marie Obenour wrote:
> > > --- a/drivers/block/loop.c
> > > +++ b/drivers/block/loop.c
> > > @@ -1205,6 +1205,12 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
> > >  	if (!part_shift)
> > >  		set_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
> > >  	mutex_lock(&lo->lo_mutex);
> > > +
> > > +	/*
> > > +	 * Increment the disk sequence number, so that userspace knows this
> > > +	 * device now points to something else.
> > > +	 */
> > > +	inc_diskseq(lo->lo_disk);
> > 
> > And I'm not sure why we even need this.  __loop_clr_fd
> > already calls disk_force_media_change, which calls inc_diskseq.
> > Why do we need an extra increment?
> 
> How does disk_force_media_change() call inc_diskseq()?  I don’t see any
> calls in the source code.  I’m going to use systemtap to see if there is
> an indirect call chain.

Were you thinking of bdev_check_media_change()?  That can call
inc_diskseq() via this call chain:

  bdev_check_media_change()
    disk_clear_events()
      disk_check_events()
        inc_diskseq()

disk_force_media_change() does not call inc_diskseq(), and I checked
that calling losetup -D does not change the diskseq of a loop device.
From what you have writte, I’m pretty sure that’s a bug in
disk_force_media_change(), though.  I’ll send a v3 that adds this call.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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