linux-kernel - Re: [PATCH] sched/completion: completion_done() should serialize with complete()

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Message-ID: <1423864582.2046.82.camel@stgolabs.net>
Date:	Fri, 13 Feb 2015 13:56:22 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, waiman.long@...com,
	raghavendra.kt@...ux.vnet.ibm.com
Subject: Re: [PATCH] sched/completion: completion_done() should serialize
 with complete()

On Thu, 2015-02-12 at 20:59 +0100, Oleg Nesterov wrote:
> Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
> reading completion state" was not correct, without lock/unlock the code
> like stop_machine_from_inactive_cpu()
> 
> 	while (!completion_done())
> 		cpu_relax();
> 
> can return before complete() finishes its spin_unlock() which writes to
> this memory. And spin_unlock_wait().

How about reverting the patch altogether?

This was never a problem nor have I ever seen a performance issues in
completions that would merit these lockless checks. The commit changelog
has *zero* information, so I don't know if this was ever a real issue.

Thanks,
Davidlohr

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