random: continually use hwgenerator randomness

The rngd kernel thread may sleep indefinitely if the entropy count is
kept above random_write_wakeup_bits by other entropy sources. To make
best use of multiple sources of randomness, mix entropy from hardware
RNGs into the pool at least once within CRNG_RESEED_INTERVAL.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -2205,13 +2205,15 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 			return;
 	}
 
-	/* Suspend writing if we're above the trickle threshold.
+	/* Throttle writing if we're above the trickle threshold.
 	 * We'll be woken up again once below random_write_wakeup_thresh,
-	 * or when the calling thread is about to terminate.
+	 * when the calling thread is about to terminate, or once
+	 * CRNG_RESEED_INTERVAL has lapsed.
 	 */
-	wait_event_interruptible(random_write_wait,
+	wait_event_interruptible_timeout(random_write_wait,
 			!system_wq || kthread_should_stop() ||
-			POOL_ENTROPY_BITS() <= random_write_wakeup_bits);
+			POOL_ENTROPY_BITS() <= random_write_wakeup_bits,
+			CRNG_RESEED_INTERVAL);
 	mix_pool_bytes(buffer, count);
 	credit_entropy_bits(entropy);
 }