Patch "pid: fix lockdep deadlock warning due to ucount_lock" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

pid: fix lockdep deadlock warning due to ucount_lock

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pid-fix-lockdep-deadlock-warning-due-to-ucount_lock.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andrei Vagin <[email protected]>
Date: Wed, 4 Jan 2017 19:28:14 -0800
Subject: pid: fix lockdep deadlock warning due to ucount_lock

From: Andrei Vagin <[email protected]>

commit add7c65ca426b7a37184dd3d2172394e23d585d6 upstream.

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
4.10.0-rc2-00024-g4aecec9-dirty #118 Tainted: G W
---------------------------------------------------------
swapper/1/0 just changed the state of lock:
(&(&sighand->siglock)->rlock){-.....}, at: [<ffffffffbd0a1bc6>] __lock_task_sighand+0xb6/0x2c0
but this lock took another, HARDIRQ-unsafe lock in the past:
(ucounts_lock){+.+...}
and interrupts could create inverse lock ordering between them.
other info that might help us debug this:

Chain exists of:                 &(&sighand->siglock)->rlock --> &(&tty->ctrl_lock)->rlock --> ucounts_lock
 Possible interrupt unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(ucounts_lock);
                               local_irq_disable();
                               lock(&(&sighand->siglock)->rlock);
                               lock(&(&tty->ctrl_lock)->rlock);
  <Interrupt>
    lock(&(&sighand->siglock)->rlock);

*** DEADLOCK ***

This patch removes a dependency between rlock and ucount_lock.

---
 kernel/pid_namespace.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

@@ -151,8 +151,12 @@ out:
 
 static void delayed_free_pidns(struct rcu_head *p)
 {
-	kmem_cache_free(pid_ns_cachep,
-			container_of(p, struct pid_namespace, rcu));
+	struct pid_namespace *ns = container_of(p, struct pid_namespace, rcu);
+
+	dec_pid_namespaces(ns->ucounts);
+	put_user_ns(ns->user_ns);
+
+	kmem_cache_free(pid_ns_cachep, ns);
 }
 
 static void destroy_pid_namespace(struct pid_namespace *ns)

@@ -162,8 +166,6 @@ static void destroy_pid_namespace(struct
 	ns_free_inum(&ns->ns);
 	for (i = 0; i < PIDMAP_ENTRIES; i++)
 		kfree(ns->pidmap[i].page);
-	dec_pid_namespaces(ns->ucounts);
-	put_user_ns(ns->user_ns);
 	call_rcu(&ns->rcu, delayed_free_pidns);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/pid-fix-lockdep-deadlock-warning-due-to-ucount_lock.patch