Re: [RFC v2 1/1] fs/namespace: defer RCU sync for MNT_DETACH umount

[Lucas Karpinski]

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