[PATCH 1/1] block: Fix elv_iosched_local_module handling of "none" scheduler

[Paolo Pisati]

From: Damien Le Moal <dlemoal at kernel.org>

BugLink: https://bugs.launchpad.net/bugs/2083845

commit e3accac1a976e65491a9b9fba82ce8ddbd3d2389 upstream.

Commit 734e1a860312 ("block: Prevent deadlocks when switching
elevators") introduced the function elv_iosched_load_module() to allow
loading an elevator module outside of elv_iosched_store() with the
target device queue not frozen, to avoid deadlocks. However, the "none"
scheduler does not have a module and as a result,
elv_iosched_load_module() always returns an error when trying to switch
to this valid scheduler.

Fix this by ignoring the return value of the request_module() call
done by elv_iosched_load_module(). This restores the behavior before
commit 734e1a860312, which was to ignore the request_module() result and
instead rely on elevator_change() to handle the "none" scheduler case.

Reported-by: Shin'ichiro Kawasaki <shinichiro.kawasaki at wdc.com>
Fixes: 734e1a860312 ("block: Prevent deadlocks when switching elevators")
Cc: stable at vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal at kernel.org>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Link: https://lore.kernel.org/r/20240917133231.134806-1-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 2327c3f9d4f7eac4939bbe2ed3206f3a7dec48e1)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 block/elevator.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/elevator.c b/block/elevator.c
index c355b55d0107..4122026b11f1 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -715,7 +715,9 @@ int elv_iosched_load_module(struct gendisk *disk, const char *buf,
 
 	strscpy(elevator_name, buf, sizeof(elevator_name));
 
-	return request_module("%s-iosched", strstrip(elevator_name));
+	request_module("%s-iosched", strstrip(elevator_name));
+
+	return 0;
 }
 
 ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
-- 
2.34.1