It seems fairly obvious that device_create_file() is not being protected
from being run concurrently on the same nbd.
Quentin found the following relevant commits:
1a2ad21 nbd: add locking to nbd_ioctl 90b8f28 [PATCH] end of methods switch: remove the old ones d4430d6 [PATCH] beginning of methods conversion 08f8585 [PATCH] move block_device_operations to blkdev.h
It would seem that the race was introduced in the process of moving nbd
from BKL to unlocked ioctls.
By setting nbd->task_recv while the mutex is held, we can prevent other
processes from running concurrently (since nbd->task_recv is also checked
while the mutex is held).
Reported-and-tested-by: Quentin Casasnovas <quentin.casasnovas@oracle.com> Cc: Markus Pargmann <mpa@pengutronix.de> Cc: Paul Clements <paul.clements@steeleye.com> Cc: Pavel Machek <pavel@suse.cz> Cc: Jens Axboe <axboe@fb.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com> Signed-off-by: Jens Axboe <axboe@fb.com>