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

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Message-ID: <20090602124854.GB9823@prithivi.gnumonks.org>
Date:	Tue, 2 Jun 2009 14:48:54 +0200
From:	Harald Welte <HaraldWelte@...tech.com>
To:	Andi Kleen <andi@...stfloor.org>
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

On Wed, May 27, 2009 at 08:08:27PM +0200, Andi Kleen wrote:
> 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.

well, they might have inadvertedly used LOCK as part of regular spinlocks,
until LOCK_PREFIX was removed, right?

> > 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.

I agree.  I was not referring to any real/known driver.  I was just trying to
figure out what kind of problem the VIA/Centaur CPU guys tried to describe when
indicating that the LOCK prefix should be used on UP to avoid DMA interfering
with read-modify-write CPU instructions.

-- 
- Harald Welte <HaraldWelte@...tech.com>	    http://linux.via.com.tw/
============================================================================
VIA Free and Open Source Software Liaison
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