linux-kernel - Re: LOCK prefix on uni processor has its use

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Message-ID: <87vdnmmnac.fsf@basil.nowhere.org>
Date:	Wed, 27 May 2009 20:08:27 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Harald Welte <HaraldWelte@...tech.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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

Harald Welte <HaraldWelte@...tech.com> writes:
> * 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

In theory yes, but not in Linux -- normal drivers simply don't use LOCK in any way
on a UP kernel.

We discussed exactly this in the earlier subthread :)

> Now the question is: Is this a valid operation of a driver?  Should the driver
> do such things, or is such a driver broken? 

The driver is broken because if it relies on this it will not work on a UP kernel.
Also it's not portable and in general a bad idea.

> 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.

Is it known which driver has this problem?

-Andi (who finds hpa's "timing theory" to be more believable anyways)

-- 
ak@...ux.intel.com -- Speaking for myself only.
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