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

[Alyssa Ross]

On Sun, Mar 19, 2023 at 11:22:25PM -0700, Christoph Hellwig wrote:
> On Sat, Mar 18, 2023 at 01:50:05AM +0000, Alyssa Ross wrote:
> > 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?
>
> Given that you're done all the hard work, and I've just reduced the
> critical section, I'd prefer to give all the credit to you.  If you're
> fine with it, I'll send out this version later:

LGTM, thanks!

> ---
> From 4648015b4193c81d3de8c1632876314b4a2ab40d Mon Sep 17 00:00:00 2001
> Subject: loop: LOOP_CONFIGURE: send uevents for partitions
>
> LOOP_CONFIGURE is, as far as I understand it, supposed to be a way to
> combine LOOP_SET_FD and LOOP_SET_STATUS64 into a single syscall.  When
> using LOOP_SET_FD+LOOP_SET_STATUS64, a single uevent would be sent for
> each partition found on the loop device after the second ioctl(), but
> when using LOOP_CONFIGURE, no such uevent was being sent.
>
> In the old setup, uevents are disabled for LOOP_SET_FD, but not for
> LOOP_SET_STATUS64.  This makes sense, as it prevents uevents being
> sent for a partially configured device during LOOP_SET_FD - they're
> only sent at the end of LOOP_SET_STATUS64.  But for LOOP_CONFIGURE,
> uevents were disabled for the entire operation, so that final
> notification was never issued.  To fix this, reduce the critical
> section to exclude the loop_reread_partitions() call, which causes
> the uevents to be issued, to after uevents are re-enabled, matching
> the behaviour of the LOOP_SET_FD+LOOP_SET_STATUS64 combination.
>
> I noticed this because Busybox's losetup program recently changed from
> using LOOP_SET_FD+LOOP_SET_STATUS64 to LOOP_CONFIGURE, and this broke
> my setup, for which I want a notification from the kernel any time a
> new partition becomes available.
>
> Signed-off-by: Alyssa Ross <hi@xxxxxxxxx>
> [hch: reduced the critical section]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Fixes: 3448914e8cc5 ("loop: Add LOOP_CONFIGURE ioctl")
> ---
>  drivers/block/loop.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>
> 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)
> --
> 2.39.2
>

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