linux-kernel - Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral infrastructure

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Message-ID: <ZKaru+Ka5kmlwrs/@lothringen>
Date:   Thu, 6 Jul 2023 13:55:39 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
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        Yair Podemsky <ypodemsk@...hat.com>
Subject: Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral
 infrastructure

On Thu, Jul 06, 2023 at 12:23:57AM +0200, Frederic Weisbecker wrote:
> Le Wed, Jul 05, 2023 at 07:12:53PM +0100, Valentin Schneider a écrit :
> +bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> +	unsigned int old, new, state;
> +	bool ret = false;
> +
> +	preempt_disable();
> +
> +	work <<= CONTEXT_WORK;
> +	state = atomic_read(&ct->state);
> +	/*
> +	 * Try setting the work until either
> +	 * - the target CPU is on the kernel
> +	 * - the work has been set
> +	 */
> +	for (;;) {
> +		/* Only set if running in user/guest */
> +		old = state;
> +		old &= ~CONTEXT_MASK;
> +		old |= CONTEXT_USER;
> +
> +		new = old | work;
> +
> +		state = atomic_cmpxchg(&ct->state, old, new);
> +		if (state & work) {

And this should be "if (state == old)", otherwise there is
a risk that someone else had set the work but atomic_cmpxchg()
failed due to other modifications is the meantime. It's then
dangerous in that case to defer the work because atomic_cmpxchg()
failures don't imply full ordering. So there is a risk that the
target executes the work but doesn't see the most recent data.

Thanks.