On Tue, 20 Dec 2005, Dinakar Guniguntala wrote:
> On Tue, Dec 20, 2005 at 02:19:56PM +0100, Ingo Molnar wrote:
> >
> > hm, i'm looking at -rf4 - these changes look fishy:
> >
> > - _raw_spin_lock(&lock_owner(lock)->task->pi_lock);
> > + if (current != lock_owner(lock)->task)
> > + _raw_spin_lock(&lock_owner(lock)->task->pi_lock);
> >
> > why is this done?
> >
>
> Ingo, this is to prevent a kernel hang due to application error.
>
> Basically when an application does a pthread_mutex_lock twice on a
> _nonrecursive_ mutex with robust/PI attributes the whole system hangs.
> Ofcourse the application clearly should not be doing anything like
> that, but it should not end up hanging the system either
>
Hmm, reading the comment on the function, wouldn't it be more natural to
use
if(task != lock_owner(lock)->task)
as it assumes that task->pi_lock is locked, not that current->pi_lock is
locked.
By the way:
task->pi_lock is taken. lock_owner(lock)->task->pi_lock will be taken.
What if the task lock_owner(lock)->task tries to lock another futex,
(lock2) with which has lock_owner(lock2)->task==task.
Can't you promote a user space futex deadlock into a kernel spin deadlock
this way?
Esben
> -Dinakar
>
>
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