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Subject: Re: [PATCH][RFC] ipc,fs: use rcu_work to free struct ipc_namespace
From:   Rik van Riel <riel@surriel.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Chris Mason <clm@fb.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        linux-fsdevel@vger.kernel.org,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>
Date:   Fri, 18 Feb 2022 11:54:21 -0500
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References: <20220217153620.4607bc28@imladris.surriel.com>
         <87iltcf996.fsf@email.froward.int.ebiederm.org>
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On Fri, 2022-02-18 at 10:08 -0600, Eric W. Biederman wrote:
>=20
> Maybe I am reading the lifetimes wrong but is there
> any chance the code can just do something like the diff below?
>=20
> AKA have a special version of kern_umount that does the call_rcu?
>=20
> Looking at rcu_reclaim_tiny I think this use of mnt_rcu is valid.
> AKA reusing the rcu_head in the rcu callback.
>=20
>=20
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 40b994a29e90..7d7aaef1592e 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4395,6 +4395,22 @@ void kern_unmount(struct vfsmount *mnt)
> =C2=A0}
> =C2=A0EXPORT_SYMBOL(kern_unmount);
> =C2=A0
> +static void rcu_mntput(struct rcu_head *head)
> +{
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct mount *mnt =3D container_of(=
head, struct mount,
> mnt_rcu);
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 mntput(&mnt->mnt);
> +}
> +
> +void kern_rcu_unmount(struct vfsmount *mnt)
> +{
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* release long term mount so mount=
 point can be released */
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (!IS_ERR_OR_NULL(mnt)) {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 struct mount *m =3D real_mount(mnt);
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 m->mnt_ns =3D NULL;
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 call_rcu(&m->mnt_rcu, rcu_mntput);
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
> +}

OK, two comments here:

1) As Paul pointed out, we need to use call_rcu_work here,
   because rcu_mntput needs to run from a work item (or well,
   any process context) because of the rwlock.
2) No user of kern_unmount can use the vfsmount structure
   after kern_unmount returns. That means they could all use
   the RCU version, and no special version is needed.

Let me go test a patch that does that...

--=20
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