linux-kernel - Re: [PATCH v5] random: defer fast pool mixing to worker

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Message-ID: <YgZ6IEbiDgz5X1ON@linutronix.de>
Date:   Fri, 11 Feb 2022 16:00:48 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v5] random: defer fast pool mixing to worker

On 2022-02-11 14:08:07 [+0100], Jason A. Donenfeld wrote:
…
> +static void mix_interrupt_randomness(struct work_struct *work)
> +{
> +	struct fast_pool *fast_pool = container_of(work, struct fast_pool, mix);
> +	unsigned long pool[ARRAY_SIZE(fast_pool->pool)];
> +	unsigned int count_snapshot;
> +	size_t i;
> +
> +	/* Check to see if we're running on the wrong CPU due to hotplug. */
> +	migrate_disable();
> +	if (fast_pool != this_cpu_ptr(&irq_randomness)) {
> +		migrate_enable();
> +		/*
> +		 * If we are unlucky enough to have been moved to another CPU,
> +		 * then we set our count to zero atomically so that when the
> +		 * CPU comes back online, it can enqueue work again. The
> +		 * _release here pairs with the atomic_inc_return_acquire in
> +		 * add_interrupt_randomness().
> +		 */
> +		atomic_set_release(&fast_pool->count, 0);
> +		return;
> +	}
> +
> +	/*
> +	 * Copy the pool to the stack so that the mixer always has a
> +	 * consistent view. It's extremely unlikely but possible that
> +	 * this 2 or 4 word read is interrupted by an irq, but in case
> +	 * it is, we double check that count stays the same.
> +	 */
> +	do {
> +		count_snapshot = (unsigned int)atomic_read(&fast_pool->count);
> +		for (i = 0; i < ARRAY_SIZE(pool); ++i)
> +			pool[i] = READ_ONCE(fast_pool->pool[i]);
> +	} while (count_snapshot != (unsigned int)atomic_read(&fast_pool->count));

Which what I wrote in the last mail, can't we just have a cmpxchg loop
here?

Sebastian