linux-kernel - Re: [rfc patch] i386: don't save eflags on task switch

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Date:	Sun, 5 Nov 2006 09:51:36 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Andi Kleen <ak@...e.de>, Zachary Amsden <zach@...are.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [rfc patch] i386: don't save eflags on task switch

On Sun, 5 Nov 2006, Arjan van de Ven wrote:
> 
> actually lockdep is pretty good at finding this type of bug IMMEDIATELY
> even without the actual race triggering ;)

Ehh. Last time this happened, lockdep didn't find _squat_. 

This was when NT and AC leaked across context switches, because the 
context switching had removed the "expensive" save/restore.

The thing is, complexity is in the unintended side effects, not in the 
code itself. For example, let's say that we changed "restore_flags()" to 
do

	static inline void restore_flags(unsigned long x)
	{
		if (x & 0x200)
			asm volatile("sti");
	}

(I didn't check that IF is 0x200, but it's something like that) and it was 
two cycles faster on average than just doing a "popf". The _complexity_ 
here is that now there might be some other x86-architecture-specific code 
sequence that nobody even _realized_ actually depended on saving the other 
flags too. Like the context switching thing did.

Is it likely? Maybe not. But that's the thing about complexity - you'd not 
know, would you?

Do a few of these kinds of things, and _individually_ they are unlikely to 
add new bugs, but once you've done ten or twenty of them, the likelihood 
that _one_ of them added a subtle bug that it will take months or years to 
find is suddenly not all that small any more.

This is why "robust" is so important. So _much_ more important than a 
cycle or two. The fact is, saving and restoring all the eflags over a 
context switch is just _more_robust_. If you do a pushfl/popfl, there's 
simply not a lot you can screw up.

			Linus
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