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

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Date:	Tue, 06 Mar 2012 14:18:22 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"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, gcc-help@....gnu.org
Subject: Re: [RFC patch] spindep: add cross cache lines checking

Resend for correct tglx's address. Sorry!
> > > 
> > > 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.
> 
> I have one concern and one questions here:
> concern: maybe the lock is in a well designed 'packed' struct, and it is
> safe for cross lines issue. but __alignof__ will return 1;
> 
> struct abc{
> 	raw_spinlock_t lock1;
> 	char 		a;
> 	char		b;
> }__attribute__((packed));
> 
> Since the lock is the first object of struct, usually it is well placed.
> 
> question: I am a idiot on gcc, I tried some parameters of gcc " --param
> l1-cache-line-size=1  -mno-align-double" and can not make a cross lines
> variable without 'packed' structure, but I still don't find a grantee
> why gcc can avoid the cross line variable if it's not in 'packed'
> structure? 
> 
> > agreed.
> > 
> > Thanks,
> > 
> > 	Ingo
> 


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