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

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Message-ID: <alpine.LFD.0.98.0706211342230.3593@woody.linux-foundation.org>
Date:	Thu, 21 Jun 2007 13:48:51 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Eric Dumazet <dada1@...mosbay.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Jarek Poplawski <jarkao2@...pl>,
	Miklos Szeredi <miklos@...redi.hu>, chris@...ee.ca,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

On Thu, 21 Jun 2007, Ingo Molnar wrote:
> 
> damn, i first wrote up an explanation about why that ugly __delay(1) is 
> there (it almost hurts my eyes when i look at it!) but then deleted it 
> as superfluous :-/

I'm fine with a delay, but the __delay(1) is simply not "correct". It 
doesn't do anything.

"udelay()" waits for a certain time. Use that. 

> the reason for the __delay(1) was really mundane: to be able to figure 
> out when to print a 'we locked up' message to the user.

No it does not.

You may think it does, but it does nothing of the sort.

Use "udelay()" or somethign that actually takes a *time*.

Just __delay() is nothing but a loop, and calling it with an argument of 1 
is stupid and buggy. 

The only *possibly* valid use of "__delay()" implies using a counter that 
is based on the "loops_per_sec" thing, which depends on what the delay  
function actually is.

For example, the delay function may well turn out to be this:

        __asm__ __volatile__(
                "\tjmp 1f\n"
                ".align 16\n"
                "1:\tjmp 2f\n"
                ".align 16\n"
                "2:\tdecl %0\n\tjns 2b"
                :"=&a" (d0)
                :"0" (loops));

Notice? "Your code, it does nothing!"

When I said that the code was buggy, I meant it.

It has nothing to do with spinlocks. And "__delay(1)" is *always* a bug.

You migth want to replace it with

	smp_rmb();
	udelay(1);

instead, at which point it *does* something: it has that read barrier 
(which is not actually needed on x86, but whatever), and it has a delay 
that is *meaningful*.

A plain "__delay(1)" is neither.

So let me repeat my statement: "What a piece of crap".

		Linus
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