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

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Message-Id: <201203050941.56502.arnd@arndb.de>
Date:	Mon, 5 Mar 2012 09:41:56 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Alex Shi <alex.shi@...el.com>
Cc:	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

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.

	Arnd
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