Re: [PATCH 12/13] loop: remove lo_refcount and avoid lo_mutex in ->open / ->release

[Christoph Hellwig <hch@lst.de>]

On Thu, Mar 24, 2022 at 06:15:18PM +0100, Christoph Hellwig wrote:
> On Thu, Mar 24, 2022 at 03:13:21PM +0100, Jan Kara wrote:
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > 
> > Looks good but I still think we need something like attached preparatory
> > patch to not regress e.g. filesystem probing triggered by udev events. What
> > do you think?
> 
> Yes, I think it makes sense to add that.

Actually, looking at it in a little more detail: this misses the
explicit kobject_uevent calls for the capacity changes.  I think the
best idea might be something like this:

---
>From db5ab8ab0fbcf07af769023a894fafc22b662cd9 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Thu, 24 Mar 2022 18:41:28 +0100
Subject: loop: suppress uevents while reconfiguring the device

Currently, udev change event is generated for a loop device before the
device is ready for IO. Due to serialization on lo->lo_mutex in
lo_open() this does not matter because anybody is able to open the
device and do IO only after the configuration is finished. However this
synchronization in lo_open() is going away so make sure userspace
reacting to the change event will see the new device state by generating
the event only when the device is setup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/block/loop.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b3170e8cdbe95..bfd21af7aa38b 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -572,6 +572,10 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 
 	if (!file)
 		return -EBADF;
+
+	/* suppress uevents while reconfiguring the device */
+	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 1);
+
 	is_loop = is_loop_device(file);
 	error = loop_global_lock_killable(lo, is_loop);
 	if (error)
@@ -626,13 +630,18 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 	fput(old_file);
 	if (partscan)
 		loop_reread_partitions(lo);
-	return 0;
+
+	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;
 
 out_err:
 	loop_global_unlock(lo, is_loop);
 out_putf:
 	fput(file);
-	return error;
+	goto done;
 }
 
 /* loop sysfs attributes */
@@ -999,6 +1008,9 @@ 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.
@@ -1101,7 +1113,12 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 		loop_reread_partitions(lo);
 	if (!(mode & FMODE_EXCL))
 		bd_abort_claiming(bdev, loop_configure);
-	return 0;
+
+	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;
 
 out_unlock:
 	loop_global_unlock(lo, is_loop);
@@ -1112,7 +1129,7 @@ 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);
-	return error;
+	goto done;
 }
 
 static void __loop_clr_fd(struct loop_device *lo, bool release)
-- 
2.30.2