linux-kernel - Re: Add memory barrier when waiting on futex

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Message-ID: <20131126082626.GB789@laptop.programming.kicks-ass.net>
Date:	Tue, 26 Nov 2013 09:26:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Ma, Xindong" <xindong.ma@...el.com>
Cc:	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"stable-commits@...r.kernel.org" <stable-commits@...r.kernel.org>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	"ccross@...gle.com" <ccross@...gle.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"dvhart@...ux.intel.com" <dvhart@...ux.intel.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"Tu, Xiaobing" <xiaobing.tu@...el.com>
Subject: Re: Add memory barrier when waiting on futex

On Tue, Nov 26, 2013 at 01:07:25AM +0000, Ma, Xindong wrote:
> I've already aware that they've protected by spinlock, this is why I adding a memory barrier to fix it.

That doesn't make sense.. the spinlocks should provide the required
serialization, there's nothing to fix.

> I reproduced this issue several times on android which running on IA dual core.

I think you need to be more specific, in particular, if spinlocks do not
provide serialization on your IA32 device, I'd call that thing broken.
Adding random memory barriers is _WRONG_.


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