Lai,
On Tue, 13 May 2014, Lai Jiangshan wrote:
> I think this patch is just a workaround, it is not the proper fix.
> you need a updated deadlock-check mechanism:
>
> - (old) skip the check when top_waiter != task_top_pi_waiter(task)
> + (new) skip the check when top_waiter->prio > task->prio
>
> /*
> * Drop out, when the task has no waiters. Note,
> * top_waiter can be NULL, when we are in the deboosting
> * mode!
> */
> if (top_waiter && (!task_has_pi_waiters(task) ||
> top_waiter != task_top_pi_waiter(task)))
> goto out_unlock_pi;
>
>
> (also need to update the code in other places respectively)
Ok, I did not think it through fully and I want to have the rtmutex
tester working again so we can check for this without going through
futex hoops and loops. I had no time yet to look into that as I needed
to understand the futex issue which exposed it first.
> On 05/13/2014 04:45 AM, Thomas Gleixner wrote:
> > /*
> > + * Deadlock check for the following scenario:
> > + *
> > + * T holds lock L and has waiters
> > + * T locks L again, but does not end up as it's own top waiter
>
> ABBA problem (TA TB TC TD are of the same priority)
>
> TA holds lock LA, and try to lock LB which TC already has waited on
> TB holds lock LB, and try to lock LA which TD already has waited on
>
> I think this check can't detect it IIUC.
>
> > + *
> > + * So we would drop out at the next check without noticing.
> > + *
> > + * Note, we need to check for orig_waiter as it might be NULL
> > + * when deboosting!
> > + */
> > + if (orig_waiter && orig_waiter->task == rt_mutex_owner(lock)) {
>
> when non-first-loop, it is already checked.
Right, but we must check it for the first loop as well. And that check
was not there ever, so it's not your problem. I verified against a
kernel w/o your optimization.
> > + ret = deadlock_detect ? -EDEADLK : 0;
> > + goto out_unlock_pi;
> > + }
>
> I considered you blamed to me.
> I would feel better if you directly blamed to me.
I blamed you as well for not following up and updating the stuff you
broke.
Thanks,
tglx
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