linux-kernel - Re: [PATCH][RFC] ipc,fs: use rcu_work to free struct ipc

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Date:   Fri, 18 Feb 2022 10:08:05 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        Chris Mason <clm@...com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-fsdevel@...r.kernel.org,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH][RFC] ipc,fs: use rcu_work to free struct ipc_namespace


Maybe I am reading the lifetimes wrong but is there
any chance the code can just do something like the diff below?

AKA have a special version of kern_umount that does the call_rcu?

Looking at rcu_reclaim_tiny I think this use of mnt_rcu is valid.
AKA reusing the rcu_head in the rcu callback.


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)
 }
 EXPORT_SYMBOL(kern_unmount);
 
+static void rcu_mntput(struct rcu_head *head)
+{
+       struct mount *mnt = container_of(head, struct mount, mnt_rcu);
+       mntput(&mnt->mnt);
+}
+
+void kern_rcu_unmount(struct vfsmount *mnt)
+{
+       /* release long term mount so mount point can be released */
+       if (!IS_ERR_OR_NULL(mnt)) {
+               struct mount *m = real_mount(mnt);
+               m->mnt_ns = NULL;
+               call_rcu(&m->mnt_rcu, rcu_mntput);
+       }
+}
+
 void kern_unmount_array(struct vfsmount *mnt[], unsigned int num)
 {
        unsigned int i;
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 5becca9be867..e54742f82e7d 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1700,7 +1700,7 @@ void mq_clear_sbinfo(struct ipc_namespace *ns)
 
 void mq_put_mnt(struct ipc_namespace *ns)
 {
-       kern_unmount(ns->mq_mnt);
+       kern_rcu_unmount(ns->mq_mnt);
 }
 
 static int __init init_mqueue_fs(void)

Eric