Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Linus Torvalds
2009-01-08T17:52:24Z

On Thu, 8 Jan 2009, Steven Rostedt wrote:
>
> We keep spinning if the owner changes.

I think we want to - if you have multiple CPU's and a heavily contended
lock that acts as a spinlock, we still _do_ want to keep spinning even if
another CPU gets the lock.

And I don't even believe that is the bug. I suspect the bug is simpler.

I think the "need_resched()" needs to go in the outer loop, or at least
happen in the "!owner" case. Because at least with preemption, what can
happen otherwise is

- process A gets the lock, but gets preempted before it sets lock->owner.

End result: count = 0, owner = NULL.

- processes B/C goes into the spin loop, filling up all CPU's (assuming
dual-core here), and will now both loop forever if they hold the kernel
lock (or have some other preemption disabling thing over their down()).

And all the while, process A would _happily_ set ->owner, and eventually
release the mutex, but it never gets to run to do either of them so.

In fact, you might not even need a process C: all you need is for B to be
on the same runqueue as A, and having enough load on the other CPU's that
A never gets migrated away. So "C" might be in user space.

I dunno. There are probably variations on the above.

Linus