On Wed, Nov 12, 2014 at 02:11:23PM +1030, Rusty Russell wrote:

Amos Kong [off-list ref] writes:

quoted

From: Rusty Russell <redacted>

current_rng holds one reference, and we bump it every time we want
to do a read from it.

This means we only hold the rng_mutex to grab or drop a reference,
so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
block on read of /dev/hwrng.

Using a kref is overkill (we're always under the rng_mutex), but
a standard pattern.

This also solves the problem that the hwrng_fillfn thread was
accessing current_rng without a lock, which could change (eg. to NULL)
underneath it.

v4: decrease last reference for triggering the cleanup

This doesn't make any sense:

quoted

+static void drop_current_rng(void)
+{
+	struct hwrng *rng = current_rng;
+
+	BUG_ON(!mutex_is_locked(&rng_mutex));
+	if (!current_rng)
+		return;
+
+	/* release current_rng reference */
+	kref_put(&current_rng->ref, cleanup_rng);
+	current_rng = NULL;
+
+	/* decrease last reference for triggering the cleanup */
+	kref_put(&rng->ref, cleanup_rng);
+}

Why would it drop the refcount twice?  This doesn't make sense.

Hmm, because you added kref_init, which initializes the reference count
to 1, you created this bug.

I saw some kernel code uses kref_* helper functions, the reference
conter is initialized to 1. Some code didn't use the helper functions
to increase/decrease the reference counter. So I will drop kref_init()
and the second kref_put().
 

Leave out the kref_init, and let it naturally be 0 (until, and if, it
becomes current_rng).  Add a comment if you want.

OK, thanks.
 

Thanks,
Rusty.
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-- 
			Amos.