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Subject: Possible kernel lock in semaphore's __down()
From: Aleksandar Dezelin <dezelin@gmail.com>
To: linux-kernel@vger.kernel.org
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Date: Wed, 29 Aug 2007 23:52:51 +0200
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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


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