Re: [PATCH v7] ath9k: let sleep be interrupted when unregistering hwrng

Hi Toke,

On Wed, Jun 29, 2022 at 11:24:49AM +0200, Toke Høiland-Jørgensen wrote:

...

• 	wait = !(filp->f_flags & O_NONBLOCK);
  

• 	if (wait && cmpxchg(&current_waiting_reader, NULL, current) != NULL) {
  

• 		err = -EINTR;
  

• 		goto out_unlock_reading;
  

• 	}
  bytes_read = rng_get_data(rng, rng_buffer,
  

• 		rng_buffer_size(),
  

• 		!(filp->f_flags & O_NONBLOCK));
  

• 		rng_buffer_size(), wait);
  

• 	if (wait && cmpxchg(&current_waiting_reader, current, NULL) != current)
  

• 		synchronize_rcu();
  

So this synchronize_rcu() is to ensure the hwrng_unregister() thread has exited the rcu_read_lock() section below? Isn't that a bit... creative... use of RCU? :)

It's to handle the extreeeeemely unlikely race in which hwrng_unregister() does its xchg, and then the thread calling rng_dev_read() entirely exits. In practice, the only way I'm able to trigger this race is by synthetically adding `msleep()` in the right spot. But anyway, for that reason, it's only synchronized if that second cmpxchg indicates that indeed the value was changed out from under us.

Also, synchronize_rcu() can potentially take a while on a busy system, is it OK to call it while holding the mutex?

The reading mutex won't be usable by anything anyway at this point, so I don't think it matters.

Jason