[ 3.5.y.z extended stable ] Patch "random: fix locking dependency with the tasklist_lock" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    random: fix locking dependency with the tasklist_lock

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 95241c5418a54c4db1c04ae1a836cad767b81942 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso at mit.edu>
Date: Mon, 4 Mar 2013 11:59:12 -0500
Subject: [PATCH] random: fix locking dependency with the tasklist_lock

commit b980955236922ae6106774511c5c05003d3ad225 upstream.

Commit 6133705494bb introduced a circular lock dependency because
posix_cpu_timers_exit() is called by release_task(), which is holding
a writer lock on tasklist_lock, and this can cause a deadlock since
kill_fasync() gets called with nonblocking_pool.lock taken.

There's no reason why kill_fasync() needs to be taken while the random
pool is locked, so move it out to fix this locking dependency.

Signed-off-by: "Theodore Ts'o" <tytso at mit.edu>
Reported-by: Russ Dill <Russ.Dill at gmail.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/char/random.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d98b2a6..91abbc4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -855,6 +855,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		      int reserved)
 {
 	unsigned long flags;
+	int wakeup_write = 0;

 	/* Hold lock while accounting */
 	spin_lock_irqsave(&r->lock, flags);
@@ -876,10 +877,8 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		else
 			r->entropy_count = reserved;

-		if (r->entropy_count < random_write_wakeup_thresh) {
-			wake_up_interruptible(&random_write_wait);
-			kill_fasync(&fasync, SIGIO, POLL_OUT);
-		}
+		if (r->entropy_count < random_write_wakeup_thresh)
+			wakeup_write = 1;
 	}

 	DEBUG_ENT("debiting %d entropy credits from %s%s\n",
@@ -887,6 +886,11 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,

 	spin_unlock_irqrestore(&r->lock, flags);

+	if (wakeup_write) {
+		wake_up_interruptible(&random_write_wait);
+		kill_fasync(&fasync, SIGIO, POLL_OUT);
+	}
+
 	return nbytes;
 }

--
1.8.1.2