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

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Date:   Fri, 18 Feb 2022 14:33:31 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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, 2022-02-18 at 19:26 +0000, Al Viro wrote:
> 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().

Fair enough. Should I make a kern_unmount_rcu() version
that gets called just from mq_put_mnt()?

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