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

Rusty Russell
2014-09-18T02:52:04Z

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