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

[Steven Rostedt]

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();
        }

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/