[PATCH 3/5] hw_random: fix unregister race.

[Rusty Russell <rusty@rustcorp.com.au>]

The previous patch added one potential problem: we can still be
reading from a hwrng when it's unregistered.  Add a wait for zero
in the hwrng_unregister path.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/char/hw_random/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index dc9092a1075d..b4a21e9521cf 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -60,6 +60,7 @@ static DEFINE_MUTEX(rng_mutex);
 static DEFINE_MUTEX(reading_mutex);
 static int data_avail;
 static u8 *rng_buffer, *rng_fillbuf;
+static DECLARE_WAIT_QUEUE_HEAD(rng_done);
 static unsigned short current_quality;
 static unsigned short default_quality; /* = 0; default to "off" */
 
@@ -98,6 +99,7 @@ static inline void cleanup_rng(struct kref *kref)
 
 	if (rng->cleanup)
 		rng->cleanup(rng);
+	wake_up_all(&rng_done);
 }
 
 static void set_current_rng(struct hwrng *rng)
@@ -529,6 +531,9 @@ void hwrng_unregister(struct hwrng *rng)
 	}
 
 	mutex_unlock(&rng_mutex);
+
+	/* Just in case rng is reading right now, wait. */
+	wait_event(rng_done, atomic_read(&rng->ref.refcount) == 0);
 }
 EXPORT_SYMBOL_GPL(hwrng_unregister);
 
-- 
1.9.1