random: Use wait_event_freezable() in add_hwgenerator_randomness()

Sebastian reports that after commit ff296293b3 ("random: Support freezable
kthreads in add_hwgenerator_randomness()") we can call might_sleep() when the
task state is TASK_INTERRUPTIBLE (state=1). This leads to the following warning.

 do not call blocking ops when !TASK_RUNNING; state=1 set at [<00000000349d1489>] prepare_to_wait_event+0x5a/0x180
 WARNING: CPU: 0 PID: 828 at kernel/sched/core.c:6741 __might_sleep+0x6f/0x80
 Modules linked in:

 CPU: 0 PID: 828 Comm: hwrng Not tainted 5.3.0-rc7-next-20190903+ #46
 RIP: 0010:__might_sleep+0x6f/0x80

 Call Trace:
  kthread_freezable_should_stop+0x1b/0x60
  add_hwgenerator_randomness+0xdd/0x130
  hwrng_fillfn+0xbf/0x120
  kthread+0x10c/0x140
  ret_from_fork+0x27/0x50

We shouldn't call kthread_freezable_should_stop() from deep within the
wait_event code because the task state is still set as
TASK_INTERRUPTIBLE instead of TASK_RUNNING and
kthread_freezable_should_stop() will try to call into the freezer with
the task in the wrong state. Use wait_event_freezable() instead so that
it calls schedule() in the right place and tries to enter the freezer
when the task state is TASK_RUNNING instead.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Keerthy <j-keerthy@ti.com>
Fixes: ff296293b3 ("random: Support freezable kthreads in add_hwgenerator_randomness()")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/random.c

@@ -327,6 +327,7 @@
 #include <linux/percpu.h>
 #include <linux/cryptohash.h>
 #include <linux/fips.h>
+#include <linux/freezer.h>
 #include <linux/ptrace.h>
 #include <linux/workqueue.h>
 #include <linux/irq.h>
@@ -2429,7 +2430,6 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 				size_t entropy)
 {
 	struct entropy_store *poolp = &input_pool;
-	bool frozen = false;
 
 	if (unlikely(crng_init == 0)) {
 		crng_fast_load(buffer, count);
@@ -2440,12 +2440,10 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	 * We'll be woken up again once below random_write_wakeup_thresh,
 	 * or when the calling thread is about to terminate.
 	 */
-	wait_event_interruptible(random_write_wait,
-			kthread_freezable_should_stop(&frozen) ||
+	wait_event_freezable(random_write_wait,
+			kthread_should_stop() ||
 			ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
-	if (!frozen) {
-		mix_pool_bytes(poolp, buffer, count);
-		credit_entropy_bits(poolp, entropy);
-	}
+	mix_pool_bytes(poolp, buffer, count);
+	credit_entropy_bits(poolp, entropy);
 }
 EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);