linux-kernel - [PATCH] random: reorder READ_ONCE() in get_random

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Date:   Fri, 30 Jun 2017 16:37:13 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Theodore Ts'o <tytso@....edu>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Jason A . Donenfeld" <Jason@...c4.com>
Subject: [PATCH] random: reorder READ_ONCE() in get_random_uXX

Since commit 4a072c71f49b ("random: silence compiler warnings and fix
race") there is a READ_ONCE().
I doubt that this required here but I don't have the time to argue with
Jason. Instead I push the access further down so we don't read crng_init
if we can leave the function after arch_get_random_XXX(). This is
something the compiler did before the READ_ONCE() got it.

Cc: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 drivers/char/random.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 01a260f67437..5e3765f6a61d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2041,7 +2041,7 @@ static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64);
 u64 get_random_u64(void)
 {
 	u64 ret;
-	bool use_lock = READ_ONCE(crng_init) < 2;
+	bool use_lock;
 	unsigned long flags = 0;
 	struct batched_entropy *batch;
 
@@ -2053,6 +2053,7 @@ u64 get_random_u64(void)
 	    arch_get_random_long((unsigned long *)&ret + 1))
 	    return ret;
 #endif
+	use_lock = READ_ONCE(crng_init) < 2;
 
 	batch = &get_cpu_var(batched_entropy_u64);
 	if (use_lock)
@@ -2073,13 +2074,14 @@ static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32);
 u32 get_random_u32(void)
 {
 	u32 ret;
-	bool use_lock = READ_ONCE(crng_init) < 2;
+	bool use_lock;
 	unsigned long flags = 0;
 	struct batched_entropy *batch;
 
 	if (arch_get_random_int(&ret))
 		return ret;
 
+	use_lock = READ_ONCE(crng_init) < 2;
 	batch = &get_cpu_var(batched_entropy_u32);
 	if (use_lock)
 		read_lock_irqsave(&batched_entropy_reset_lock, flags);
-- 
2.13.2