[PATCH] random: fix locking dependency with the tasklist_lock

[Theodore Ts'o]

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@xxxxxxx>
Reported-by: Russ Dill <Russ.Dill@xxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
 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 85e81ec..57d4b15 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -852,6 +852,7 @@ 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);
@@ -873,10 +874,8 @@ 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 %zu entropy credits from %s%s\n",
@@ -884,6 +883,11 @@ 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;
 }
 
-- 
1.7.12.rc0.22.gcdd159b

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