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

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Date:	Mon, 04 May 2015 11:26:31 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
CC:	Rik van Riel <riel@...hat.com>,
	"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 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.

Paolo
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