linux-kernel - Re: LOCK prefix on uni processor has its use (was Re: [BUG FIX] Make x86

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Date:	Wed, 27 May 2009 19:56:59 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Harald Welte <HaraldWelte@...tech.com>
CC:	lkml@...ethan.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: LOCK prefix on uni processor has its use (was Re: [BUG FIX] Make
 x86_32 uni-processor Atomic ops, Atomic)

Harald Welte wrote:
> * A read-modify-write sequence cannot be interupted.
> * All X86 instructions except rep-strings are atomic wrt interrupts.
> * The lock prefix has uses on a UP processor: It keeps DMA devices from
>   interfering with a read-modify-write sequence

Correct.

> Now the question is: Is this a valid operation of a driver?  Should the driver
> do such things, or is such a driver broken?  When would that occur?  I'm trying
> to come up with a case, but typically you e.g. allocate some DMA buffer and
> then don't touch it until the hardware has processed it.

The Linux driver model does not permit this as a *lot* of hardware
doesn't support this correctly, and even on x86 there are lots of
chipset bugs in this regard.  It is of course possible to write x86-only
drivers that would do this anyway, but those should not use LOCK_PREFIX
instructions.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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