Re: [PATCH] loop: defer the queue limits clear to a workqueue

[Bart Van Assche]

On 8/28/26 12:20 AM, Tao Cui wrote:
> loop_clear_limits() calls queue_limits_commit_update() directly from
> the loop workqueue that processes the request.  That does a
> non-atomic struct assignment to q->limits without freezing the queue,
> which races with lockless readers of q->limits on other CPUs - bio
> splitting reads max_hw_sectors, the discard path reads
> max_hw_discard_sectors - and can let them observe torn values.  The
> trigger is a discard or write-zeroes request on a loop device whose
> backing file does not support the corresponding fallocate operation.
>
> The code already has an XXX comment saying this should move to a
> workqueue.  Do that: schedule a work item on the system workqueue,
> where it is safe to freeze the queue and update the limits using
> queue_limits_commit_update_frozen().  Accumulate pending modes in
> lo->clear_limits_mode so that failures between scheduling and
> execution of the work item are not lost, and cancel the work item
> before the device is freed.  If the device is reconfigured to a
> backing file that does support the operation in that window, the
> stale clear takes effect and discard is disabled until the next
> reconfiguration.
Please help with reviewing this patch, which seems more complete to me
than this patch:
https://lore.kernel.org/linux-block/c2ab2547-63b3-48cf-87c1-fc53219e360a@xxxxxxxxxxxxxxxxxxx/

Thanks,

Bart.