Re: [PATCH][RFC]: mutex: adaptive spin

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, 6 Jan 2009, Linus Torvalds wrote:
> > 
> > So it should be renamed. Something like "task_is_oncpu()" or whatever.
> 
> Another complaint, which is tangentially related in that it actually 
> concerns "current".
> 
> Right now, if some process deadlocks on a mutex, we get hung process, 
> but with a nice backtrace and hopefully other things (that don't need 
> that lock) still continue to work.
> 
> But if I read it correctly, the adaptive spin code will instead just 
> hang. Exactly because "task_is_current()" will also trigger for that 
> case, and now you get an infinite loop, with the process spinning until 
> it looses its own CPU, which obviously will never happen.
> 
> Yes, this is the behavior we get with spinlocks too, and yes, lock 
> debugging will talk about it, but it's a regression. We've historically 
> had a _lot_ more bad deadlocks on mutexes than we have had on spinlocks, 
> exactly because mutexes can be held over much more complex code. So 
> regressing on it and making it less debuggable is bad.
> 
> IOW, if we do this, then I think we need a
> 
> 	BUG_ON(task == owner);
> 
> in the waiting slow-path. I realize the test already exists for the 
> DEBUG case, but I think we just want it even for production kernels. 
> Especially since we'd only ever need it in the slow-path.

yeah, sounds good.

One thought:

BUG_ON()'s do_exit() shows a slightly misleading failure pattern to users: 
instead of a 'hanging' task, we'd get a misbehaving app due to one of its 
tasks exiting spuriously. It can even go completely unnoticed [users dont 
look at kernel logs normally] - while a hanging task generally does get 
noticed. (because there's no progress in processing)

So instead of the BUG_ON() we could emit a WARN_ONCE() perhaps, plus not 
do any spinning and just block - resulting in an uninterruptible task 
(that the user will probably notice) and a scary message in the syslog? 
[all in the slowpath]

So in this case WARN_ONCE() is both more passive (it does not run 
do_exit()), and shows the more intuitive failure pattern to users. No 
strong feelings though.

	Ingo