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Date:   Fri, 1 Feb 2019 09:26:48 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrea Parri <andrea.parri@amarulasolutions.com>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2] sched: Use READ_ONCE()/WRITE_ONCE() in
 move_queued_task()/task_rq_lock()
Message-ID: <20190201082647.GA31498@hirez.programming.kicks-ass.net>
References: <20190121155240.27173-1-andrea.parri@amarulasolutions.com>
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On Mon, Jan 21, 2019 at 04:52:40PM +0100, Andrea Parri wrote:
> move_queued_task() synchronizes with task_rq_lock() as follows:
> 
> 	move_queued_task()		task_rq_lock()
> 
> 	[S] ->on_rq = MIGRATING		[L] rq = task_rq()
> 	WMB (__set_task_cpu())		ACQUIRE (rq->lock);
> 	[S] ->cpu = new_cpu		[L] ->on_rq
> 
> where "[L] rq = task_rq()" is ordered before "ACQUIRE (rq->lock)" by an
> address dependency and, in turn, "ACQUIRE (rq->lock)" is ordered before
> "[L] ->on_rq" by the ACQUIRE itself.
> 
> Use READ_ONCE() to load ->cpu in task_rq() (c.f., task_cpu()) to honor
> this address dependency.  Also, mark the accesses to ->cpu and ->on_rq
> with READ_ONCE()/WRITE_ONCE() to comply with the LKMM.
> 
> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>

Thanks!