linux-kernel - Re: [PATCH] x86_64 : support atomic ops with 64 bits integer values

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Date:	Sat, 16 Aug 2008 11:43:31 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64 : support atomic ops with 64 bits integer values

* H. Peter Anvin (hpa@...or.com) wrote:
> Mathieu Desnoyers wrote:
>> x86_64 add/sub atomic ops does not seems to accept integer values bigger
>> than 32 bits as immediates. Intel's add/sub documentation specifies they
>> have to be passed as registers.
>
> This is correct; this is in fact true for all instructions except "mov".
>
> Whether it's sign- or zero-extending is sometimes subtle, but not in these 
> cases.
>
> Do you happen to know if this is a manifest bug in the current kernel (i.e. 
> if there is anywhere we're using more than ±2 GB as a constant to these 
> functions?)
>

No, I did not hit this on current kernel code and the effect is quite
esasy to detect : the assembler spits an error.

I have hit this problem when tying to implement a better rwlock design
than is currently in the mainline kernel (I know the RT kernel has a
hard time with rwlocks), and had to play with add/sub of large values.
The idea is to bring down the interrupt latency caused by rwlocks shared
between fast read-side interrupt handlers and slow thread context
read-sides (tasklist_lock is the perfect example). In that case, the
worse case interrupt latency is caused by the irq-disabled writer lock
when contended by the slow readers. I will probably post a RFC about
this in a near future.

Mathieu

> Either way, I'll queue this up to tip:x86/urgent if Ingo hasn't already 
> since this is a pure bug fix.
>
> 	-hpa

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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