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

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Date:	Thu, 21 Jun 2007 22:09:41 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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


* 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. This used to trigger with the ->tree_lock rwlock i think, on 
heavy MM loads. The starvation got so bad that the NMI watchdog started 
triggering ...

interestingly, this only triggered for certain rwlocks. Thus we, after a 
few failed attempts to pacify this open-coded loop, currently have that 
code disabled in lib/spinlock_debug.c:

 #if 0           /* This can cause lockups */
 static void __write_lock_debug(rwlock_t *lock)
 {
         u64 i;
         u64 loops = loops_per_jiffy * HZ;
         int print_once = 1;

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

the weird thing is that we still have the _very same_ construct in 
__spin_lock_debug():

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

if there are any problems with this then people are not complaining loud 
enough :-)

note that because this is a trylock based loop, the acquire+release 
sequence problem should not apply to this problem.

	Ingo
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