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Message-ID: <5f442a7770fe4ac06b2837e4f937d559f5d17b8b.camel@surriel.com>
Subject: Re: [PATCH 1/2] vfs: free vfsmount through rcu work from
 kern_unmount
From:   Rik van Riel <riel@surriel.com>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        linux-fsdevel@vger.kernel.org, paulmck@kernel.org,
        gscrivan@redhat.com, Eric Biederman <ebiederm@xmission.com>,
        Chris Mason <clm@fb.com>
Date:   Fri, 18 Feb 2022 14:33:31 -0500
In-Reply-To: <Yg/y6qv6dZ2fc5z1@zeniv-ca.linux.org.uk>
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         <20220218183114.2867528-2-riel@surriel.com>
         <Yg/y6qv6dZ2fc5z1@zeniv-ca.linux.org.uk>
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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.
> >=20
> > 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().
>=20
> NAK.=C2=A0 There's code that relies upon kern_unmount() being
> synchronous.=C2=A0 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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