Re: Real-Time Preemption and RCU

[Manfred Spraul]

Thomas Gleixner wrote:

> On Sun, 2005-03-20 at 07:36 +0100, Manfred Spraul wrote:
>  
>
> > cpu 1:
> > acquire random networking spin_lock_bh()
> >
> > cpu 2:
> > read_lock(&tasklist_lock) from process context
> > interrupt. softirq. within softirq: try to acquire the networking lock.
> > * spins.
> >
> > cpu 1:
> > hardware interrupt
> > within hw interrupt: signal delivery. tries to acquire tasklist_lock.
> >
> > --> deadlock.
> >    
>
> Signal delivery from hw interrupt context (interrupt is flagged
> SA_NODELAY) is not possible in RT preemption mode. The
> local_irq_save_nort() check in __cache_alloc will catch you.
>
>  
That was just one random example.
Another one would be :

drivers/chat/tty_io.c, __do_SAK() contains
   read_lock(&tasklist_lock);
   task_lock(p);

kernel/sys.c, sys_setrlimit contains
   task_lock(current->group_leader);
   read_lock(&tasklist_lock);

task_lock is a shorthand for spin_lock(&p->alloc_lock). If read_lock is 
a normal spinlock, then this is an A/B B/A deadlock.

--
   Manfred
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