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Date:   Thu, 19 Jan 2023 22:09:26 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Eric Chanudet <echanude@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-rt-users@vger.kernel.org,
        Alexander Larsson <alexl@redhat.com>,
        Andrew Halaney <ahalaney@redhat.com>
Subject: Re: [RFC PATCH RESEND 1/1] fs/namespace: defer free_mount from
 namespace_unlock
Message-ID: <Y8m/ljQUJOefsD6O@ZenIV>
References: <20230119211455.498968-1-echanude@redhat.com>
 <20230119211455.498968-2-echanude@redhat.com>
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On Thu, Jan 19, 2023 at 04:14:55PM -0500, Eric Chanudet wrote:
> From: Alexander Larsson <alexl@redhat.com>
> 
> Use call_rcu to defer releasing the umount'ed or detached filesystem
> when calling namepsace_unlock().
> 
> Calling synchronize_rcu_expedited() has a significant cost on RT kernel
> that default to rcupdate.rcu_normal_after_boot=1.
> 
> For example, on a 6.2-rt1 kernel:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>            0.07464 +- 0.00396 seconds time elapsed  ( +-  5.31% )
> 
> With this change applied:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>         0.00162604 +- 0.00000637 seconds time elapsed  ( +-  0.39% )
> 
> Waiting for the grace period before completing the syscall does not seem
> mandatory. The struct mount umount'ed are queued up for release in a
> separate list and no longer accessible to following syscalls.

Again, NAK.  If a filesystem is expected to be shut down by umount(2),
userland expects it to have been already shut down by the time the
syscall returns.

It's not just visibility in namespace; it's "can I pull the disk out?".
Or "can the shutdown get to taking the network down?", for that matter.