Re: [PATCH] block: don't release queue's sysfs lock during switching elevator

[Jens Axboe <axboe@kernel.dk>]

On 9/23/19 5:12 PM, Ming Lei wrote:
> cecf5d87ff20 ("block: split .sysfs_lock into two locks") starts to
> release & acquire sysfs_lock before registering/un-registering elevator
> queue during switching elevator for avoiding potential deadlock from
> showing & storing 'queue/iosched' attributes and removing elevator's
> kobject.
> 
> Turns out there isn't such deadlock because 'q->sysfs_lock' isn't
> required in .show & .store of queue/iosched's attributes, and just
> elevator's sysfs lock is acquired in elv_iosched_store() and
> elv_iosched_show(). So it is safe to hold queue's sysfs lock when
> registering/un-registering elevator queue.
> 
> The biggest issue is that commit cecf5d87ff20 assumes that concurrent
> write on 'queue/scheduler' can't happen. However, this assumption isn't
> true, because kernfs_fop_write() only guarantees that concurrent write
> aren't called on the same open file, but the write could be from
> different open on the file. So we can't release & re-acquire queue's
> sysfs lock during switching elevator, otherwise use-after-free on
> elevator could be triggered.
> 
> Fixes the issue by not releasing queue's sysfs lock during switching
> elevator.

Applied, thanks Ming.

-- 
Jens Axboe