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@alyssa.is>
[hch: reduced the critical section]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: 3448914e ("loop: Add LOOP_CONFIGURE ioctl")
Link: https://lore.kernel.org/r/20230320125430.55367-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

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)