linux-kernel - Re: [RFC patch] spindep: add cross cache lines checking

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Date:	Mon, 5 Mar 2012 11:43:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Alex Shi <alex.shi@...el.com>, tglx@...utronix.com,
	"mingo@...hat.com" <mingo@...hat.com>, hpa@...or.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, andi.kleen@...el.com
Subject: Re: [RFC patch] spindep: add cross cache lines checking


* Arnd Bergmann <arnd@...db.de> wrote:

> On Monday 05 March 2012, Alex Shi wrote:
> > Subject: [PATCH] lockdep: add cross cache lines checking
> > 
> > Modern x86 CPU won't hold whole memory bus when executing 
> > 'lock' prefixed instructions unless the instruction 
> > destination is crossing 2 cache lines. If so, it is disaster 
> > of system performance.
> > 
> > Actually if the lock is not in the 'packed' structure, gcc 
> > places it safely under x86 arch. But seems add this checking 
> > in CONFIG_DEBUG_LOCK_ALLOC is harmless.
> 
> Have you tried making this a compile-time check using 
> __alignof__? I would say that any spinlock in a packed data 
> structure is basically a bug, even more so on most other 
> architectures besides x86.

agreed.

Thanks,

	Ingo
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