linux-kernel - Re: [RFC v2 1/1] fs/namespace: defer RCU sync for MNT

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Date: Tue, 30 Apr 2024 09:25:55 -0400
From: Lucas Karpinski <lkarpins@...hat.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: brauner@...nel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, alexl@...hat.com, echanude@...hat.com, ikent@...hat.com, 
	ahalaney@...hat.com
Subject: Re: [RFC v2 1/1] fs/namespace: defer RCU sync for MNT_DETACH umount

On Fri, Apr 26, 2024 at 09:09:41PM +0100, Al Viro wrote:
> > +		call_rcu(&drelease->rcu, delayed_mount_release);
> 
> ... which is a bad idea, since call_rcu() callbacks are run
> from interrupt context.  Which makes blocking in them a problem.
>

Thanks for the quick review.

Documentation/RCU/checklist.rst suggests switching to queue_rcu_work()
function in scenarios where the callback function can block. This seems
like it would fix the issue you found, while still providing similar
performance improvements.

workqueue:
perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
        0.003066 +- 0.000307 seconds time elapsed  ( +- 10.02% )

callrcu:
perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
        0.0030812 +- 0.0000524 seconds time elapsed  ( +-  1.70% )

Regards,
Lucas