linux-kernel - Re: [RFC PATCH] sched, timer: Use atomics for thread_group

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Message-ID: <20150123092508.GJ2896@worktop.programming.kicks-ass.net>
Date:	Fri, 23 Jan 2015 10:25:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer
 stats

On Thu, Jan 22, 2015 at 07:31:53PM -0800, Jason Low wrote:
> +static void update_gt_cputime(struct thread_group_cputimer *a, struct task_cputime *b)
>  {
> +	if (b->utime > atomic64_read(&a->utime))
> +		atomic64_set(&a->utime, b->utime);
>  
> +	if (b->stime > atomic64_read(&a->stime))
> +		atomic64_set(&a->stime, b->stime);
>  
> +	if (b->sum_exec_runtime > atomic64_read(&a->sum_exec_runtime))
> +		atomic64_set(&a->sum_exec_runtime, b->sum_exec_runtime);
>  }

See something like this is not safe against concurrent adds.
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