Possible kernel lock in semaphore's __down()

Aleksandar Dezelin
2007-08-29T21:53:04Z

Hi!

I'm a newbie here on the list and also as a "kernel hacker". There's a
bug reported in bugzilla (Bug 7927), cite:

> In the function __down
>
> fastcall void __sched __down(struct semaphore * sem)
> {
> struct task_struct *tsk = current;
> DECLARE_WAITQUEUE(wait, tsk);
> unsigned long flags;
>
> tsk->state = TASK_UNINTERRUPTIBLE;
> spin_lock_irqsave(&sem->wait.lock, flags);
> add_wait_queue_exclusive_locked(&sem->wait, &wait);
> ...
> }
>
>
> From this code fragment, it sets the tsk->state to TASK_UNINTERRUPTIBLE before
> gets the spinlock. Assume at that moment, a interrupt ocuur and and after the
> interrupt handle ends, an other process is scheduled to run (assume the kernel
> is preemptalbe). In this case, the previous process ( its state has set to
> TASK_UNINTERRUPTIBLE) has been picked off the run queue, and it has not yet add
> to the wait queue( sem->wait ), so it may be never waited up forever.
>

I have marked it as rejected as as I can see at the time this function is called,
it is guaranteed that ret_from_intr() will not call schedule() on return from an
interrupt handler to either kernel space or user space because of the call
to macro might_sleep() in semaphore's down(). Am I wrong?

Thanks and best regards,
Aleksandar Dezelin