Re: [Xenomai-core] [BUG] module_put called over non-root domain

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Hi Philippe,
>>
>> Jan Kiszka wrote:
>>
>>> Just ran into this with CONFIG_IPIPE_DEBUG_CONTEXT (maybe due to some
>>> bug of my own):
>>
>> Here is some code to trigger the issue reliably:
>>
>> #include <sys/mman.h>
>> #include <native/task.h>
>>
>> void task_fnct(void *arg)
>> {
>>         rt_task_delete(NULL);
>> }
>>
>> main()
>> {
>>         RT_TASK task;
>>         mlockall(MCL_CURRENT|MCL_FUTURE);
>>         rt_task_spawn(&task, "task", 0, 10, 0, task_fnct, NULL);
>> }
>>
>>
>>
>>> [  102.616000] I-pipe: Detected illicit call from domain 'Xenomai'
>>> [  102.616000]         into a service reserved for domain 'Linux' and below.
>>> [  102.616000]        c741bdc8 00000000 00000000 c8860ef8 c741bdec c0105683 c032c200 c13fe22c
>>> [  102.616000]        c0361f00 c741be08 c01519ed c032f5b8 c032c742 c03380b3 c8885100 c78beac0
>>> [  102.616000]        c741be14 c0142ce9 c7a80b30 c741be3c c884d075 c885f150 c8860ef8 c741be3c
>>> [  102.616000] Call Trace:
>>> [  102.616000]  [<c0104d9f>] show_trace_log_lvl+0x1f/0x40
>>> [  102.616000]  [<c0104e71>] show_stack_log_lvl+0xb1/0xe0
>>> [  102.616000]  [<c0105683>] show_stack+0x33/0x40
>>> [  102.616000]  [<c01519ed>] ipipe_check_context+0xad/0xc0
>>> [  102.616000]  [<c0142ce9>] module_put+0x19/0x90
>>> [  102.616000]  [<c884d075>] xnshadow_unmap+0xb5/0x130 [xeno_nucleus]
>>> [  102.616000]  [<c8871dc5>] __shadow_delete_hook+0x25/0x30 [xeno_native]
>>> [  102.616000]  [<c8842f78>] xnpod_schedule+0xb58/0x12f0 [xeno_nucleus]
>>> [  102.616000]  [<c8844bfb>] xnpod_delete_thread+0x2cb/0x3d0 [xeno_nucleus]
>>> [  102.616000]  [<c886f5bd>] rt_task_delete+0x20d/0x220 [xeno_native]
>>>
>>> I would dare to say that module_put in xnshadow_unmap is not well placed
>>> as it can wakeup a Linux process. The module ref-counter maintenance
>>> needs some postponing, I guess.
>>
>> Attached is a patch proposal. It solves the issue by postponing the
>> module_put via a new schedule_linux_call. Note that this approach issues
>> LO_WAKEUP_REQ where the old test (p->state != TASK_RUNNING) would not
>> have done so. I don't see negative side effects yet, and I'm furthermore
>> not sure of the old code was handling SMP scenarios safely (What if the
>> thread to be unmapped was running on different CPU than xnshadow_unmap?
>> How to ensure test-atomicity then?).
> 
> This one counts as mine! I am Ok with the fix, but IMHO, the
> "if(p->state != TASK_RUNNING)" probably has a reason, so I would leave
> it in the new implementation.
> 

But then I need some official declaration, that this test cannot race
with whatever in case the target task runs on another CPU. I tried to
direct my brain along this twisted path, but stopped before it started
hurting too much (others would call it laziness). :)

Jan


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