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

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Date:	Fri, 26 Oct 2007 22:39:07 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Bart Van Assche" <bart.vanassche@...il.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Andrew Haley" <aph@...hat.com>
Subject: Re: Is gcc thread-unsafe?

Alan Cox <alan@...rguk.ukuu.org.uk> writes:
>
> You can stop the compiler but not the CPU - and some processors will
> certainly speculatively load across conditionals, reorder writes etc

The difference is that the CPU knows how to cancel most[1] side effects
of these speculative accesses (e.g. by not issuing exceptions[2] etc.). 

The compiler doesn't normally (except on some architectures with special support like IA64;
but I'm not sure gcc supports it there) 

[1] In some it can't and we've had problems with that in the past. e.g. in
a few cases speculative reads can be a problem. But we generally fix or
workaround those cases in the code.
[2] Modulo hardware bugs -- see the hall of shame in x86_64 fault.c

-Andi
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