linux-kernel - Re: [PATCH v2] loop: LOOP_CONFIGURE: send uevents for partitions

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Date:   Sat, 18 Mar 2023 01:50:05 +0000
From:   Alyssa Ross <hi@...ssa.is>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        Martijn Coenen <maco@...roid.com>, linux-block@...r.kernel.org
Subject: Re: [PATCH v2] loop: LOOP_CONFIGURE: send uevents for partitions

On Wed, Mar 15, 2023 at 08:48:40AM -0700, Christoph Hellwig wrote:
> On Sun, Mar 12, 2023 at 07:10:31PM +0000, Alyssa Ross wrote:
> > +	 * Now that we are done, reread the partitions with uevent
> > +	 * re-enabled if appropriate to let userspace know about the
> > +	 * changes.
> > +	 */
> > +	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), !partscan_uevent);
> > +	if (partscan)
> > +		loop_reread_partitions(lo);
> > +	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
>
> What worries me here is that you move the partition re-read out of
> the exclusive claim, which is another potentially user visible
> change (and user visible behavior changes are a field of landmines
> in loop as you have noticed).

Makes sense.

> But in the end we only need to suppress the events until Lo_Bound
> is set.  So something like the patch below that reduces the no even
> critical section might do the job?

If you say so!  I had trouble understanding which parts of the function
uevents needed to be suppressed for, so I was trying to move as little
as possible out of that section.

What happens next?  I'm still getting up to speed on the kernel
development process — will you submit this as a patch with a patch body
and a S-o-b?  Or am I supposed to do something with it?

I know enough to know that I should give you a:

Tested-by: Alyssa Ross <hi@...ssa.is>

> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 839373451c2b7d..9d61c027185141 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1010,9 +1010,6 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
>  	/* This is safe, since we have a reference from open(). */
>  	__module_get(THIS_MODULE);
>
> -	/* suppress uevents while reconfiguring the device */
> -	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 1);
> -
>  	/*
>  	 * If we don't hold exclusive handle for the device, upgrade to it
>  	 * here to avoid changing device under exclusive owner.
> @@ -1067,6 +1064,9 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
>  		}
>  	}
>
> +	/* suppress uevents while reconfiguring the device */
> +	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 1);
> +
>  	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
>  	set_disk_ro(lo->lo_disk, (lo->lo_flags & LO_FLAGS_READ_ONLY) != 0);
>
> @@ -1109,17 +1109,17 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
>  	if (partscan)
>  		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
>
> +	/* enable and uncork uevent now that we are done */
> +	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
> +
>  	loop_global_unlock(lo, is_loop);
>  	if (partscan)
>  		loop_reread_partitions(lo);
> +
>  	if (!(mode & FMODE_EXCL))
>  		bd_abort_claiming(bdev, loop_configure);
>
> -	error = 0;
> -done:
> -	/* enable and uncork uevent now that we are done */
> -	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
> -	return error;
> +	return 0;
>
>  out_unlock:
>  	loop_global_unlock(lo, is_loop);
> @@ -1130,7 +1130,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
>  	fput(file);
>  	/* This is safe: open() is still holding a reference. */
>  	module_put(THIS_MODULE);
> -	goto done;
> +	return error;
>  }
>
>  static void __loop_clr_fd(struct loop_device *lo, bool release)

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