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Date: Mon, 04 May 2015 09:30:47 -0400
From: Rik van Riel <riel@redhat.com>
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To: Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, williams@redhat.com,
        Andrew Lutomirski <luto@kernel.org>, fweisbec@redhat.com,
        Peter Zijlstra <peterz@infradead.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry
References: <554399D1.6010405@redhat.com> <20150501155912.GA451@gmail.com> <CALCETrVZf11EYLhKWOfeQSyzq9eq5KB+btcY19JF+sJvs2zMXA@mail.gmail.com> <20150501162109.GA1091@gmail.com> <5543A94B.3020108@redhat.com> <20150501163431.GB1327@gmail.com> <5543C05E.9040209@redhat.com> <20150501184025.GA2114@gmail.com> <5543CFE5.1030509@redhat.com> <CALCETrX2Di19atf4+Nx5cCOxbqoFdx+USLJ-WRSBy_Se25RE-g@mail.gmail.com> <20150502052733.GA9983@gmail.com> <55473B47.6080600@redhat.com>
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On 05/04/2015 05:26 AM, Paolo Bonzini wrote:
> 
> 
> On 02/05/2015 07:27, Ingo Molnar wrote:
>>
>>   - synchronize_rcu() avoids having to send an IPI by taking a 
>>     peak at rq->curr's pt_regs::flag, and if:
>>
>>      - the flag is 0 then it has observed a quiescent state.
>>
>>      - the flag is 1, then it would set TIF_NOHZ and wait for a 
>>        completion from a TIF_NOHZ callback.
> 
> Isn't this racy?
> 
> 	synchronize_rcu CPU		nohz CPU
> 	---------------------------------------------------------
> 					set flag = 0
> 	read flag = 0
> 					return to userspace
> 	set TIF_NOHZ
> 
> and there's no guarantee that TIF_NOHZ is ever processed by the nohz CPU.

If the flag and TIF_NOHZ live in the same word, we can cmpxchg
and be guaranteed atomicity.

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