[PATCH 2/2] random: spread out jitter callback to different CPUs

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Rather than merely hoping that the callback gets called on another CPU,
arrange for that to actually happen, by round robining which CPU the
timer fires on. This way, on multiprocessor machines, we exacerbate
jitter by touching the same memory from multiple different cores.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index fdf15f5c87dd..74627b53179a 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1209,6 +1209,7 @@ static void __cold try_to_generate_entropy(void)
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
+	int cpu = -1;
 
 	for (i = 0; i < NUM_TRIAL_SAMPLES - 1; ++i) {
 		stack.entropy = random_get_entropy();
@@ -1223,8 +1224,17 @@ static void __cold try_to_generate_entropy(void)
 	stack.samples = 0;
 	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);
+		if (!timer_pending(&stack.timer)) {
+			preempt_disable();
+			do {
+				cpu = cpumask_next(cpu, cpu_online_mask);
+				if (cpu == nr_cpumask_bits)
+					cpu = cpumask_first(cpu_online_mask);
+			} while (cpu == smp_processor_id() && cpumask_weight(cpu_online_mask) > 1);
+			stack.timer.expires = jiffies;
+			add_timer_on(&stack.timer, cpu);
+			preempt_enable();
+		}
 		mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 		schedule();
 		stack.entropy = random_get_entropy();
-- 
2.37.3