Re: Livelock with the shmctl04 test program from linux test project

[Manfred Spraul <manfred@colorfullife.com>]

Alexander Nyberg wrote:

>Sorry for late reply, but I just can't understand why & how this happens, been trying to grasp
>the IPC/SHM part but I'm missing something. One processor gets locked up and never released.
>  
>
Ok - that's a deadlock.

>I did:
>
>printk("taking lock\n"); 
>spin_lock(&info->lock);
>printk("lock taken\n");
>
>and it never prints out "lock taken" so i know where it locks up. Now the fun part,
>spinlock debugging doesn't catch it,
>
That's not surprising: the full debug code is only active for 
uniprocessor kernels. On SMP, only a simple check for unitialized 
spinlocks is performed.
Btw, I'd use
    printk("thread %d, struct %p: taking lock", current->pid, info);
Then you are certain that you are not fooled by multiple concurrent 
operations.

> but I did a simple patch to show who is holding a lock
>at the current time, and it appears noone has taken the lock. I really don't get this.
>
>  
>
I must think about it. Who's printed as the last owner that released the 
lock? Perhaps there is a race with segment destruction: The structures 
are protected by RCU.

Could you enable debug spinlocks and slab debugging? I would have 
expected an error message from spinlock debugging due to bad magic.

--
    Manfred