linux-kernel - Re: [PATCH 2/2] ipc/sem.c: Remove duplicated memory barriers.

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Date:	Wed, 13 Jul 2016 20:37:33 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Michael Ellerman <mpe@...erman.id.au>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, 1vier1@....de,
	felixh@...ormatik.uni-bremen.de
Subject: Re: [PATCH 2/2] ipc/sem.c: Remove duplicated memory barriers.

Hi Davidlohr,

On 07/13/2016 06:16 PM, Davidlohr Bueso wrote:
> Manfred, shouldn't this patch be part of patch 1 (as you add the 
> unnecessary barriers there? Iow, can we have a single patch for all this?
>
Two reasons:
- patch 1 is safe for backporting, patch 2 not.
- patch 1 is safe on all architectures, for patch 2 I would like to get 
some review feedback.

e.g. I just found/read 51d7d5205d33 ("powerpc: Add smp_mb() to 
arch_spin_is_locked()"):
For powerpc, a smp_mb() was added into spin_is_locked(), more or less 
for ipc/sem.c.

Patch 1 replaces the spin_is_locked() with smp_load_acquire().
Isn't that the proof that smp_mb() is required?



--
     Manfred