Re: [PATCH 0/3] Fix a deadlock related to modifying queue attributes

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 08, 2025 at 09:11:41AM -0700, Bart Van Assche wrote:
> I will look into modifying the SRP tests in the blktests repository such
> that these use bio-based mode instead of request-based mode.

Note that this just fixes the test case.  The fact that request
based dm-multipath keeps active requests and thus an elevated 
q_usage_counter around still exists then, with effects both to sysfs
and other users of queue freezing.

> So the question remains what to do about these two regressions:
> * The deadlock triggered by modifying a sysfs attribute of a
>   dm-multipath device configured with "queue_if_no_path" and no paths
>   (temporarily).

That's not a deadlock by the classic definition, but yes, it is hang
that should be addressed.

> * Slower booting of Linux devices that modify sysfs attributes
>   synchronously during boot.


So which attributes are regularly modified?  Note that for read-ahead
it should be safe to drop the freeze as unlike the others it is not
used for splitting I/O to the limits accepted by the hardware.  So
we could probably drop the freeze IFF the patch documents that it is
safe.  But it still won't fix the root cause.