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DateSun, 20 Mar 2005 17:57:23 +0100
FromManfred Spraul <>
SubjectRe: Real-Time Preemption and RCU
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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