linux-kernel - [PATCH 1/2] random: schedule jitter credit for next jiffy, not in two jiffies

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Date:   Sat,  1 Oct 2022 01:10:49 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sultan Alsawaf <sultan@...neltoast.com>
Subject: [PATCH 1/2] random: schedule jitter credit for next jiffy, not in two jiffies

Counterintuitively, mod_timer(..., jiffies + 1) will cause the timer to
fire not in the next jiffy, but in two jiffies. The way to cause
the timer to fire in the next jiffy is with mod_timer(..., jiffies).
Doing so then lets us bump the upper bound back up again.

Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Sultan Alsawaf <sultan@...neltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 64ee16ffb8b7..fdf15f5c87dd 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1205,7 +1205,7 @@ static void __cold entropy_timer(struct timer_list *timer)
  */
 static void __cold try_to_generate_entropy(void)
 {
-	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
+	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
@@ -1224,7 +1224,7 @@ static void __cold try_to_generate_entropy(void)
 	timer_setup_on_stack(&stack.timer, entropy_timer, 0);
 	while (!crng_ready() && !signal_pending(current)) {
 		if (!timer_pending(&stack.timer))
-			mod_timer(&stack.timer, jiffies + 1);
+			mod_timer(&stack.timer, jiffies);
 		mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 		schedule();
 		stack.entropy = random_get_entropy();
-- 
2.37.3