Amos Kong <akong@xxxxxxxxxx> writes:
> From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>
> 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:
> +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(¤t_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.
Leave out the kref_init, and let it naturally be 0 (until, and if, it
becomes current_rng). Add a comment if you want.
Thanks,
Rusty.
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