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

On Thu, 8 Jan 2009, Linus Torvalds wrote:
>
> 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.

Ouch! I think you are on to something:

for (;;) {
struct thread_info *owner;
old_val = atomic_cmpxchg(&lock->count, 1, 0);
if (old_val == 1) {
lock_acquired(&lock->dep_map, ip);
mutex_set_owner(lock);
return 0;
}
if (old_val < 0 && !list_empty(&lock->wait_list))
break;
/* See who owns it, and spin on him if anybody */
owner = ACCESS_ONCE(lock->owner);
The owner was preempted before assigning lock->owner (as you stated).

if (owner && !spin_on_owner(lock, owner))
break;
We just spin :-(

I think adding the:

+ if (need_resched())
+ break;
would solve the problem.

Thanks,

-- Steve


cpu_relax();
}


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