linux-kernel - Re: Is gcc thread-unsafe?

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Message-ID: <4728FD4C.2010509@cfl.rr.com>
Date:	Wed, 31 Oct 2007 18:10:20 -0400
From:	Phillip Susi <psusi@....rr.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kleen, Andi" <ak@...e.de>
Subject: Re: Is gcc thread-unsafe?

>             pushl   %ebp
>             movl    %esp, %ebp
>             cmpl    $0, 8(%ebp)
>             movl    $1, %eax
>             cmove   v, %eax        ; load (maybe)
>             movl    %eax, v        ; store (always)
>             popl    %ebp
>             ret

How is this even an optimization?  It looks SLOWER to me.  The 
conditional read wastes memory bandwidth sometimes, if the condition is 
true, and v isn't already in the cache.  The unconditional write wastes 
memory bandwidth ALL the time, and dirties/flushes caches, in addition 
to not being thread safe.

This SHOULD be using a conditional write instead of a conditional read 
and an unconditional write.

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