linux-kernel - Re: [BUG] long freezes on thinkpad t60

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Date:	Mon, 18 Jun 2007 11:25:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Miklos Szeredi <miklos@...redi.hu>, cebbert@...hat.com,
	chris@...ee.ca, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

On Mon, 18 Jun 2007, Ingo Molnar wrote:
> 
> ok. Do we have an guarantee that cpu_relax() is also an smp_rmb()?

The common use for cpu_relax() is basically for code that does

	while (*ptr != val)
		cpu_relax();

so yes, an architecture that doesn't notice writes by other CPU's on its 
own had *better* have an implied read barrier in its "cpu_relax()" 
implementation. For example, the irq handling code does

	while (desc->status & IRQ_INPROGRESS)
		cpu_relax();

which is explicitly about waiting for another CPU to get out of their 
interrupt handler. And one classic use for it in drivers is obviously the

	while (time_before (jiffies, next))
		cpu_relax();

kind of setup (and "jiffies" may well be updated on another CPU: the fact 
that it is "volatile" is just a *compiler* barrier just like cpu_relax() 
itself will also be, not a "smp_rmb()" kind of hardware barrier).

So we could certainly add the smp_rmb() to make it more explicit, and it 
wouldn't be *wrong*.

But quite frankly, I'd personally rather not - if it were to make a 
difference in some situation, it would just be papering over a bug in 
cpu_relax() itself.

The whole point of cpu_relax() is about busy-looping, after all. And the 
only thing you really *can* busy-loop on in a CPU is basically a memory 
value.

So the smp_rmb() would I think distract from the issue, and at best paper 
over some totally separate bug.

> hm, this might still go into a non-nice busy loop on SMP: one cpu runs 
> the strace, another one runs two tasks, one of which is runnable but not 
> on the runqueue (the one we are waiting for). In that case we'd call 
> yield() on this CPU in a loop

Sure. I agree - we can get into a loop that calls yield(). But I think a 
loop that calls yield() had better be ok - we're explicitly giving the 
scheduler the ability to to schedule anything else that is relevant. 

So I think yield()'ing is fundamentally different from busy-looping any 
other way. 

Would it be better to be able to have a wait-queue, and actually *sleep* 
on it, and not even busy-loop using yield? Yeah, possibly. I cannot 
personally bring myself to care about that kind of corner-case situation, 
though.

			Linus
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