linux-kernel - Re: [PATCH 1/2] vfs: free vfsmount through rcu work from kern

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Message-ID: <Yg/y6qv6dZ2fc5z1@zeniv-ca.linux.org.uk>
Date:   Fri, 18 Feb 2022 19:26:34 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        linux-fsdevel@...r.kernel.org, paulmck@...nel.org,
        gscrivan@...hat.com, Eric Biederman <ebiederm@...ssion.com>,
        Chris Mason <clm@...com>
Subject: Re: [PATCH 1/2] vfs: free vfsmount through rcu work from kern_unmount

On Fri, Feb 18, 2022 at 01:31:13PM -0500, Rik van Riel wrote:
> After kern_unmount returns, callers can no longer access the
> vfsmount structure. However, the vfsmount structure does need
> to be kept around until the end of the RCU grace period, to
> make sure other accesses have all gone away too.
> 
> This can be accomplished by either gating each kern_unmount
> on synchronize_rcu (the comment in the code says it all), or
> by deferring the freeing until the next grace period, where
> it needs to be handled in a workqueue due to the locking in
> mntput_no_expire().

NAK.  There's code that relies upon kern_unmount() being
synchronous.  That's precisely the reason why MNT_INTERNAL
is treated that way in mntput_no_expire().