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Date:   Mon, 11 Nov 2019 09:43:13 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC PATCH 4/4] irq_work: Weaken ordering in irq_work_run_list()
Message-ID: <20191111084313.GN4131@hirez.programming.kicks-ass.net>
References: <20191108160858.31665-1-frederic@kernel.org>
 <20191108160858.31665-5-frederic@kernel.org>
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On Fri, Nov 08, 2019 at 05:08:58PM +0100, Frederic Weisbecker wrote:

> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 49c53f80a13a..b709ab05cbfd 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -34,8 +34,8 @@ static bool irq_work_claim(struct irq_work *work)
>  	oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->flags);
>  	/*
>  	 * If the work is already pending, no need to raise the IPI.
> +	 * The pairing atomic_andnot() followed by a barrier in irq_work_run()
> +	 * makes sure everything we did before is visible.
>  	 */
>  	if (oflags & IRQ_WORK_PENDING)
>  		return false;

> @@ -151,14 +151,16 @@ static void irq_work_run_list(struct llist_head *list)
>  		 * to claim that work don't rely on us to handle their data
>  		 * while we are in the middle of the func.
>  		 */
> -		flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->flags);
> +		atomic_andnot(IRQ_WORK_PENDING, &work->flags);
> +		smp_mb__after_atomic();

I think I'm prefering you use:

		flags = atomic_fetch_andnot_acquire(IRQ_WORK_PENDING, &work->flags);

Also, I'm cursing at myself for the horrible comments here.

>  		work->func(work);
>  		/*
>  		 * Clear the BUSY bit and return to the free state if
>  		 * no-one else claimed it meanwhile.
>  		 */
> -		(void)atomic_cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
> +		(void)atomic_cmpxchg(&work->flags, flags & ~IRQ_WORK_PENDING,
> +				     flags & ~IRQ_WORK_CLAIMED);
>  	}
>  }
>  
> -- 
> 2.23.0
>