linux-kernel - Re: [PATCH] mark read_crX() asm code as volatile

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Message-ID: <4703342E.3020101@zytor.com>
Date:	Tue, 02 Oct 2007 23:18:22 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Chuck Ebbert <cebbert@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Kirill Korotaev <dev@...nvz.org>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH] mark read_crX() asm code as volatile

Nick Piggin wrote:
>>
>> This should work because the result gets used before reading again:
>>
>> read_cr3(a);
>> write_cr3(a | 1);
>> read_cr3(a);
>>
>> But this might be reordered so that b gets read before the write:
>>
>> read_cr3(a);
>> write_cr3(a | 1);
>> read_cr3(b);
>>
>> ?
> 
> I don't see how, as write_cr3 clobbers memory.

Because read_cr3() doesn't depend on memory, and b could be stored in a 
register.

	-hpa
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