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
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