linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Message-ID: <55477487.8050209@redhat.com>
Date:	Mon, 04 May 2015 09:30:47 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry

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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