linux-kernel - Re: [PATCH] random: Support freezable kthreads in add_hwgenerator

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Date:   Wed, 04 Sep 2019 11:49:57 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        linux-crypto@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
        Keerthy <j-keerthy@...com>, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] random: Support freezable kthreads in add_hwgenerator_randomness()

Quoting Sebastian Andrzej Siewior (2019-09-04 04:00:38)
> On 2019-08-22 15:55:19 [+1000], Herbert Xu wrote:
> > Patch applied.  Thanks.
> [ ff296293b3538 ("random: Support freezable kthreads in add_hwgenerator_randomness()") ]
> 
> and since kthread_freezable_should_stop() has might_sleep() in it, I get
> this:
> 
> |: 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
> 

Ugh ok. Thanks for the report.

We're getting warnings because the task is in TASK_INTERRUPTIBLE state
when we call kthread_freezable_should_stop() from deep within the wait
event code. We shouldn't do that, and instead we should call
wait_event_freezable() and kthread_should_stop() in the condition. This
way we'll call into the freezer when the task is woken up by the suspend
path.

Can you try this?

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9b54cdb301d3..d3beed084c0a 100644
--- a/drivers/char/random.c
+++ b/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);
@@ -2440,13 +2440,11 @@ 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);