random: fix locking dependency with the tasklist_lock

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@mit.edu>
Reported-by: Russ Dill <Russ.Dill@gmail.com>
Cc: stable@kernel.org

1 files changed, 8 insertions, 4 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 85e81ec1451e..57d4b152267c 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;
 }