linux-kernel - Re: [RFC] Control dependencies

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Message-ID: <528F9B8D.2090806@hurleysoftware.com>
Date:	Fri, 22 Nov 2013 12:59:41 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	"Figo.zhang" <figo1802@...il.com>
Subject: Re: [RFC] Control dependencies

On 11/22/2013 08:46 AM, Peter Zijlstra wrote:
> How about the below version?
>
> ---
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -61,19 +61,20 @@ static void perf_output_put_handle(struc
>   	 *
>   	 *   kernel				user
>   	 *
> -	 *   READ ->data_tail			READ ->data_head
> -	 *   smp_mb()	(A)			smp_rmb()	(C)
> -	 *   WRITE $data			READ $data
> -	 *   smp_wmb()	(B)			smp_mb()	(D)
> -	 *   STORE ->data_head			WRITE ->data_tail
> +	 *   if (LOAD ->data_tail) {		LOAD ->data_head
> +	 *			(A)		smp_rmb()	(C)
> +	 *	STORE $data			LOAD $data
> +	 *	smp_wmb()	(B)		smp_mb()	(D)
> +	 *	STORE ->data_head		STORE ->data_tail


I wasn't subscribed to linux-arch so missed the smp_store_release()
outcome, if there was one.

Are (B) and (D) still slated for changing to STORE.rel semantics,
aka smp_store_release()?

I realize that, for the perf ring buffer, (D) is in userspace but
I'm also interested in non-perf situations where (D) would be in the
kernel.

Regards,
Peter Hurley



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