linux-kernel - [PATCH 3/3] random: use trylock in irq handler rather than spinning

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Message-Id: <20220212231022.679926-4-Jason@zx2c4.com>
Date:   Sun, 13 Feb 2022 00:10:22 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux@...inikbrodowski.net
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>
Subject: [PATCH 3/3] random: use trylock in irq handler rather than spinning

crng_pre_init_inject() (and prior crng_fast_load()) uses a trylock when
in fast mode, so that it never contends. We should be doing the same
when grabbing a spinlock for mixing into the entropy pool. So switch to
doing that before calling the underscored _mix_pool_bytes().

Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9a8e1bb9845d..ca224c3f2561 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1286,13 +1286,10 @@ void add_interrupt_randomness(int irq)
 			atomic_set(&fast_pool->count, 0);
 			fast_pool->last = now;
 
-			/*
-			 * Technically this call means that we're using a spinlock_t
-			 * in the IRQ handler, which isn't terrific for PREEMPT_RT.
-			 * However, this only happens during boot, and then never
-			 * again, so we live with it.
-			 */
-			mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+			if (spin_trylock(&input_pool.lock)) {
+				_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+				spin_unlock(&input_pool.lock);
+			}
 		}
 		return;
 	}
-- 
2.35.0