Amos Kong <akong@xxxxxxxxxx> writes:
> From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>
> 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
That's a bit weird. The usual pattern would be to hold a reference
until we're actually finished, but this reference is a bit weird.
We hold the mutex across cleanup, so we could grab that but we have to
take care sleeping inside wait_event, otherwise Peter will have to fix
my code again :)
AFAICT the wake_woken() stuff isn't merged yet, so your patch will
have to do for now.
> @@ -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);
> +
The atomic_read() isn't necessary here.
However, you should probably init cleanup_done in hwrng_register().
(Probably noone does unregister then register, but let's be clear).
Thanks,
Rusty.
> }
> 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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