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

[Tao Cui <cui.tao@linux.dev>]

Hi Bart,

在 2026/9/1 00:25, Bart Van Assche 写道:
> On 8/28/26 12:20 AM, Tao Cui wrote:
>> 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().
> The patch description explains why queue freezing is necessary but
> does not add queue freeze and unfreeze calls. Is that perhaps an
> oversight?
> 

Good question, I should have spelled this out in the commit
message.  The freeze and unfreeze are inside
queue_limits_commit_update_frozen() itself (block/blk-settings.c):

	memflags = blk_mq_freeze_queue(q);
	ret = queue_limits_commit_update(q, lim);
	blk_mq_unfreeze_queue(q, memflags);

The naming is a bit counterintuitive: the _frozen variant is the
one that freezes the queue itself, while the plain
queue_limits_commit_update() expects the caller to have frozen the
queue already ("The caller must have frozen the queue or ensure
that there are no outstanding I/Os by other means").  The in-tree
callers use it that way too - blk_integrity_unregister() and the
sd.c revalidation paths call the _frozen variant without freezing
the queue themselves.

The old loop_clear_limits() was exactly a caller that did not meet
that expectation, since it ran in the loop workqueue where the
queue cannot be frozen, so moving to the work item and using the
_frozen wrapper is the fix.

Thanks,
Tao

> Thanks,
> 
> Bart.