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

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Date:	Thu, 21 Jun 2007 13:14:52 -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:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > If somebody can actually come up with a sequence where we have 
> > spinlock starvation, and it's not about an example of bad locking, and 
> > nobody really can come up with any other way to fix it, we may 
> > eventually have to add the notion of "fair spinlocks".
> 
> there was one bad case i can remember: the spinlock debugging code had a 
> trylock open-coded loop and on certain Opterons CPUs were starving each 
> other.

But this is a perfect example of exactly what I'm talking about:

 THAT CODE IS HORRIBLY BUGGY!

It's not the spinlocks that are broken, it's that damn code.

>          for (;;) {
>                  for (i = 0; i < loops; i++) {
>                          if (__raw_write_trylock(&lock->raw_lock))
>                                  return;
>                          __delay(1);
>                  }

What a piece of crap. 

Anybody who ever waits for a lock by busy-looping over it is BUGGY, 
dammit!

The only correct way to wait for a lock is:

  (a) try it *once* with an atomic r-m-w 
  (b) loop over just _reading_ it (and something that implies a memory 
      barrier, _not_ "__delay()". Use "cpu_relax()" or "smp_rmb()")
  (c) rinse and repeat.

and code like the above should just be shot on sight.

So don't blame the spinlocks or the hardware for crap code.

		Linus
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