linux-kernel - [PATCH] random: Remove preempt disabled region

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Message-Id: <20180711143721.18378-1-bigeasy@linutronix.de>
Date:   Wed, 11 Jul 2018 16:37:21 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        tglx@...utronix.de, Ingo Molnar <mingo@...nel.org>,
        Ingo Molnar <mingo@...e.hu>, "Theodore Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] random: Remove preempt disabled region

From: Ingo Molnar <mingo@...e.hu>

No need to keep preemption disabled across the whole function.

mix_pool_bytes() uses a spin_lock() to protect the pool and there are
other places like write_pool() whhich invoke mix_pool_bytes() without
disabling preemption.
credit_entropy_bits() is invoked from other places like
add_hwgenerator_randomness() without disabling preemption.

Before commit 95b709b6be49 ("random: drop trickle mode") the function
used __this_cpu_inc_return() which would require disabled preemption.
The preempt_disable() section was added in commit 43d5d3018c37 ("[PATCH]
random driver preempt robustness", history tree).  It was claimed that
the code relied on "vt_ioctl() being called under BKL".

Cc: "Theodore Ts'o" <tytso@....edu>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[bigeasy: enhance the commit message]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 drivers/char/random.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index cd888d4ee605..e43690977b29 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1122,8 +1122,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	} sample;
 	long delta, delta2, delta3;
 
-	preempt_disable();
-
 	sample.jiffies = jiffies;
 	sample.cycles = random_get_entropy();
 	sample.num = num;
@@ -1161,8 +1159,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	 * and limit entropy entimate to 12 bits.
 	 */
 	credit_entropy_bits(r, min_t(int, fls(delta>>1), 11));
-
-	preempt_enable();
 }
 
 void add_input_randomness(unsigned int type, unsigned int code,
-- 
2.18.0