netdev - Re: [PATCH v2 18/33] mnt: support ns lookup

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Message-ID: <20250916044648.GP39973@ZenIV>
Date: Tue, 16 Sep 2025 05:46:48 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
	linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>,
	Jeff Layton <jlayton@...nel.org>, Mike Yuan <me@...dnzj.com>,
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
	Lennart Poettering <mzxreary@...inter.de>,
	Daan De Meyer <daan.j.demeyer@...il.com>,
	Aleksa Sarai <cyphar@...har.com>, Jens Axboe <axboe@...nel.dk>,
	Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>, linux-nfs@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH v2 18/33] mnt: support ns lookup

On Tue, Sep 16, 2025 at 04:59:49AM +0100, Al Viro wrote:
> On Tue, Sep 16, 2025 at 04:56:33AM +0100, Al Viro wrote:
> > 	if (!RB_EMPTY_NODE(to_ns_common(ns)->ns_tree_node))
> 
>  	if (!RB_EMPTY_NODE(&to_ns_common(ns)->ns_tree_node))
> 
> obviously...

FWIW, how about the following - I put the commit below into never-rebased
branch, pull it into #work.mount and you do the same to your branch
just prior to 18/33?  The difference from one in #work.mount is that
this variant checks RB_EMPTY_NODE(&ns->mnt_ns_tree_node) instead of
list_empty(&ns->mnt_ns_list).  The reasons why it's safe lockless are
pretty much the same...

Objections?  Does vfs/vfs.git #no-rebases-mnt_ns_tree_remove look sane
for you?

mnt_ns_tree_remove(): DTRT if mnt_ns had never been added to mnt_ns_list

Actual removal is done under the lock, but for checking if need to bother
the lockless RB_EMPTY_NODE() is safe - either that namespace had never
been added to mnt_ns_tree, in which case the the node will stay empty, or
whoever had allocated it has called mnt_ns_tree_add() and it has already
run to completion.  After that point RB_EMPTY_NODE() will become false and
will remain false, no matter what we do with other nodes in the tree.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/fs/namespace.c b/fs/namespace.c
index ae6d1312b184..39afeb521a80 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -187,7 +187,7 @@ static void mnt_ns_release_rcu(struct rcu_head *rcu)
 static void mnt_ns_tree_remove(struct mnt_namespace *ns)
 {
 	/* remove from global mount namespace list */
-	if (!is_anon_ns(ns)) {
+	if (!RB_EMPTY_NODE(&ns->mnt_ns_tree_node)) {
 		mnt_ns_tree_write_lock();
 		rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
 		list_bidir_del_rcu(&ns->mnt_ns_list);