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Date:   Sat, 19 Feb 2022 05:50:35 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Rik van Riel <riel@surriel.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        linux-fsdevel@vger.kernel.org, paulmck@kernel.org,
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Subject: Re: [PATCH 1/2] vfs: free vfsmount through rcu work from kern_unmount
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On Fri, Feb 18, 2022 at 09:06:31PM +0000, Al Viro wrote:

> FWIW, that won't work correctly wrt failure exits.  I'm digging
> through the lifetime rules in there right now, will post when
> I'm done.

OK, now that I'd reconstructed the picture...  The problems with
delayed shutdown are prevented by mq_clear_sbinfo() call in there -
mqueue is capable of more or less gracefully dealing with
having ->s_fs_info ripped from under it, which is what that
thing does.  Before the kern_unmount().  And since that code is
non-modular, we don't need to protect that either.

IOW, having
void put_ipc_ns(struct ipc_namespace *ns)
{
        if (refcount_dec_and_lock(&ns->ns.count, &mq_lock)) {
		mq_clear_sbinfo(ns);
		spin_unlock(&mq_lock);
		free_ipc_ns(ns);
	}
}

and
void mq_put_mnt(struct ipc_namespace *ns)
{
	/*
	 * The only reason it's safe to have the mntput async
	 * is that we'd already ripped the ipc_namespace away
	 * from the mqueue superblock, by having called
	 * mq_clear_sbinfo().
	 *
	 * NOTE: kern_unmount_rcu() IS NOT SAFE TO USE
	 * WITHOUT SERIOUS PRECAUTIONS.
	 *
	 * Anything that is used by filesystem must either
	 * be already taken away (and fs must survive that)
	 * or have its destruction delayed until the superblock
	 * shutdown.
	 *
	 */
        kern_unmount_rcu(ns->mq_mnt);
}

would suffice.  free_ipc_work/free_ipc/mnt_llist can be killed off.