Messages in this thread | | | Date | Mon, 09 Jan 2006 12:01:35 -0800 | From | David Singleton <> | Subject | Re: robust futex deadlock detection patch |
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Esben Nielsen wrote:
> >I am a little bit confused when I read check_futex_deadlock(): >It takes a parameter struct thread_info *ti and immediately do >struct task_struct *task = ti->task. Now we have the usual pair >(thread_info *ti, task_t *task) corresponding to the same process. Later >on in the function you do ti = lock_owner(lock), but do not update task. >Was this intented? > >
Whoops. You are right. I've fixed the update to the task structure. The check_futex_deadlock() code now mirrors the existing check_deadlock() code.
>Anyway, I can't see that you have locked the necesary raw_spin_locks. >Forinstance lock_owner(lock) must be called with the lock->wait_lock taken >and task->blocked_on needs task->pi_lock locked. > >
Actually those locks are grabbed in the down_try_futex code. I hold both of those locks across the check for deadlocks and into __down_interruptible. Those locks need to be held for the check for deadlocks and holding them from down_try_futex to down_interruptible garuantees that a thread that enters the kernel to block on a lock will block on the lock. There is no window any more between dropping the robust_sem and mmap_sem and calling down_interruptible.
This also fixed an SMP problem that Dave Carlson has been seeing on earlier patches.
The new patch is at
http://source.mvista.com/~dsingleton/patch-2.6.15-rt2-rf2
David
>To avoid deadlocks in all the deadlock detection you have to do the loop >something like > >for(owner = current; owner; ) { > raw_spin_lock(&owner->pi_lock); > if(owner->task->blocked_on) { > lock = owner->task->blocked_on->lock; > raw_spin_lock(&lock->wait_lock); > owner2 = lock_owner(lock); > if(owner2) { > get_task_struct(owner2->task); > raw_spin_unlock(&lock->wait_lock) > raw_spin_unlock(&owner->pi_lock); > } > put_task_struct(owner->task); > owner = owner2; > if(owner2==current) DEADLOCK >} > > >Esben > > > > >>David >> >>- >>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >>the body of a message to majordomo@vger.kernel.org >>More majordomo info at http://vger.kernel.org/majordomo-info.html >>Please read the FAQ at http://www.tux.org/lkml/ >> >> >> > > > > >
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