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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, "Theodore Tso" <tytso@mit.edu>,
        Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH 4.9 033/264] random: remove preempt disabled region
Date:   Thu, 23 Jun 2022 18:40:26 +0200
Message-Id: <20220623164345.005711217@linuxfoundation.org>
In-Reply-To: <20220623164344.053938039@linuxfoundation.org>
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From: Ingo Molnar <mingo@elte.hu>

commit b34fbaa9289328c7aec67d2b8b8b7d02bc61c67d upstream.

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@mit.edu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bigeasy: enhance the commit message]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/char/random.c |    4 ----
 1 file changed, 4 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1189,8 +1189,6 @@ static void add_timer_randomness(struct
 	} sample;
 	long delta, delta2, delta3;
 
-	preempt_disable();
-
 	sample.jiffies = jiffies;
 	sample.cycles = random_get_entropy();
 	sample.num = num;
@@ -1228,8 +1226,6 @@ static void add_timer_randomness(struct
 	 * 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,