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Subject[PATCH v4 4/6] hw_random: fix unregister race.
Date
From: 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.

v4: add cleanup_done flag to insure that cleanup is done

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Amos Kong <akong@redhat.com>
---
drivers/char/hw_random/core.c | 8 ++++++++
include/linux/hw_random.h | 1 +
2 files changed, 9 insertions(+)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 27ad6b4..c31bf91 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 +99,8 @@ static inline void cleanup_rng(struct kref *kref)

if (rng->cleanup)
rng->cleanup(rng);
+ rng->cleanup_done = true;
+ wake_up_all(&rng_done);
}

static void set_current_rng(struct hwrng *rng)
@@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng)
kthread_stop(hwrng_fill);
} else
mutex_unlock(&rng_mutex);
+
+ /* Just in case rng is reading right now, wait. */
+ wait_event(rng_done, rng->cleanup_done &&
+ atomic_read(&rng->ref.refcount) == 0);
+
}
EXPORT_SYMBOL_GPL(hwrng_unregister);

diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index c212e71..7832e50 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -46,6 +46,7 @@ struct hwrng {
/* internal. */
struct list_head list;
struct kref ref;
+ bool cleanup_done;
};

/** Register a new Hardware Random Number Generator driver. */
--
1.9.3


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