linux-kernel - [PATCH RFC fs/namespace] Make kern_unmount() use synchronize_rcu

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Message-ID: <20220214190549.GA2815154@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 14 Feb 2022 11:05:49 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     clm@...com
Cc:     riel@...riel.com, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel-team@...com
Subject: [PATCH RFC fs/namespace] Make kern_unmount() use
 synchronize_rcu_expedited()

Experimental.  Not for inclusion.  Yet, anyway.

Freeing large numbers of namespaces in quick succession can result in
a bottleneck on the synchronize_rcu() invoked from kern_unmount().
This patch applies the synchronize_rcu_expedited() hammer to allow
further testing and fault isolation.

Hey, at least there was no need to change the comment!  ;-)

Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: <linux-fsdevel@...r.kernel.org>
Cc: <linux-kernel@...r.kernel.org>
Not-yet-signed-off-by: Paul E. McKenney <paulmck@...nel.org>

---

 namespace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 40b994a29e90d..79c50ad0ade5b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4389,7 +4389,7 @@ void kern_unmount(struct vfsmount *mnt)
 	/* release long term mount so mount point can be released */
 	if (!IS_ERR_OR_NULL(mnt)) {
 		real_mount(mnt)->mnt_ns = NULL;
-		synchronize_rcu();	/* yecchhh... */
+		synchronize_rcu_expedited();	/* yecchhh... */
 		mntput(mnt);
 	}
 }