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

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 85e81ec1451e38accca94d8dee9d4703ff614063..57d4b152267cc419ad0b123cadadaccd256dd96f 100644 (file)
--- 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;
 }