Re: [PATCH v2 3/6] hw_random: use reference counts on each struct hwrng.

Amos Kong [off-list ref] writes:

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.

V2: reduce reference count in signal_pending path

OK, I changed it to do the put_rng before the check, so instead of:

We have:


		mutex_unlock(&reading_mutex);
		put_rng(rng);

		if (need_resched())
			schedule_timeout_interruptible(1);

		if (signal_pending(current)) {
			err = -ERESTARTSYS;
			goto out;
		}
	}
out:
	return ret ? : err;

out_unlock_reading:
	mutex_unlock(&reading_mutex);
	put_rng(rng);
	goto out;
}


Cheers,
Rusty.