On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote:

Amos Kong [off-list ref] writes:

quoted

From: Rusty Russell <redacted>

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.

The cleanup function is a callback function of kref_put(), we can't
use the same reference count inside cleanup function.
 

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 :)

We didn't hold rng_mutex inside cleanup_rng(), am I missing something?
 

AFAICT the wake_woken() stuff isn't merged yet, so your patch will
have to do for now.

Can you provide some patches/mail link here? I searched nothing about wake_woken.
 

quoted

@@ -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).

Got it.
 

Thanks,
Rusty.

quoted

 }
 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

-- 
			Amos.