Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Gilles Chanteperdrix wrote:
>>>>>> Jan Kiszka wrote:
>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>> Jan Kiszka wrote:
>>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>>> Jan Kiszka wrote:
>>>>>>>>>>> Hi Gilles,
>>>>>>>>>>>
>>>>>>>>>>> I'm pushing your findings to the list, also as my colleagues showed
>>>>>>>>>>> strong interest - this thing may explain rare corruptions for us as
>>>>>>>>>>> well.
>>>>>>>>>>>
>>>>>>>>>>> I thought a bit about that likely u_mode-related crash in your test
>>>>>>>>>>> case
>>>>>>>>>>> and have the following theory so far: If the xeno_current_mode
>>>>>>>>>>> storage
>>>>>>>>>>> is allocated on the application heap (!HAVE_THREAD, that's also
>>>>>>>>>>> what we
>>>>>>>>>>> are forced to use), it is automatically freed on thread termination
>>>>>>>>>>> in
>>>>>>>>>>> the context of the dying thread. If the thread is already migrated
>>>>>>>>>>> to
>>>>>>>>>>> secondary or if that happens while it is cleaned up (i.e. before
>>>>>>>>>>> calling
>>>>>>>>>>> for exit into the kernel), there is no problem, Xenomai will not
>>>>>>>>>>> touch
>>>>>>>>>>> the mode storage anymore. But if the thread happens to delete the
>>>>>>>>>>> storage "silently", without any migration, the final exit will
>>>>>>>>>>> trigger
>>>>>>>>>>> one further access. And that takes place against an invalid head
>>>>>>>>>>> area at
>>>>>>>>>>> this point.
>>>>>>>>>>>
>>>>>>>>>>> Does this make sense?
>>>>>>>>>> Yes, it is the issue we observed.
>>>>>>>>>>
>>>>>>>>>>> If that is true, all we need to do is to force a migration before
>>>>>>>>>>> releasing the mode storage. Could you check this?
>>>>>>>>>> No, that does not fly. Calling, for instance,
>>>>>>>>>> __wrap_pthread_mutex_lock
>>>>>>>>>> in another TSD cleanup function is which could be called after the
>>>>>>>>>> current_mode TSD cleanup is allowed and could trigger a switch to
>>>>>>>>>> primary mode and a write to the u_mode.
>>>>>>>>>>
>>>>>>>>> Good point. Mmh. Another, but ABI-breaking, way would be to add a
>>>>>>>>> syscall for deregistering the u_mode pointer...
>>>>>>>> That is the thing we did to verify that we had this bug. But this
>>>>>>>> syscall would be also called too soon, and suffers from the TSD cleanup
>>>>>>>> functions order again.
>>>>>>>>
>>>>>>> Right, the only complete fix without losing functionality is to add an
>>>>>>> option to our ABI for requesting kernel-managed memory if dynamic
>>>>>>> allocation is necessary (i.e. no TLS is available).
>>>>>> No. TLS may as well suffer from the same issue, since it is handled by
>>>>>> the glibc or libgcc, over which we have no control. So yes, it may work
>>>>>> by chance today, but may as well stop working tomorrow. We use
>>>>>> kernel-managed memory all the time, final point.
>>>>> I think we are still in the solution finding process, no need for early
>>>>> conclusions.
>>>>>
>>>>> See, we actually do not need kernel-managed storage for u_mode at all.
>>>>> u_mode is an optimization, mostly for our fast user space mutexes. We
>>>>> can indeed switch off all updates by the kernel and will still be able
>>>>> to provide all required features - just less optimally. Adding a third
>>>>> state, "invalid", we can make all mutex users assume they need the slow
>>>>> syscall path on uncontended acquisition. And assert_nrt will probably be
>>>>> happy about a syscall replacement for u_mode when it became invalid.
>>>>>
>>>>> This invalid state (maybe u_mode == -1 with TLS, and mode_key == NULL
>>>>> without it) is entered during thread clean up with the help of a TSD
>>>>> destructor. The destructor will then deregister our u_mode storage from
>>>>> the kernel so that it doesn't matter if we release the memory
>>>>> immediately and explicitly (w/o TLS) or leave this to glibc (/w TLS).
>>>>> And in this model, it also doesn't matter when precisely the destructor
>>>>> is called.
>>>> We have to add a syscall to propagate this value to kernel-space, and
>>>> clutter the kernel-space code which uses u_mode with tests to see if
>>>> u_mode is valid or not, and we have to clutter the code which uses
>>>> u_mode in user-space to handle that invalid state. And every time we add
>>>> a user of u_mode, we have to think about the invalid state. A lot of
>>>> clutter.
>>>>
>>>> The two last issues may be removed by handling the invalid state only in
>>>> the function which returns the current mode. If the state is invalid,
>>>> then issue the syscall. Admittedly, we get two syscalls for mutex locks,
>>>> but who cares.
>>>>
>>>> However, what for? Allocating u_mode in the process private sem_heap, as
>>>> I suggest since the beggining, looks so much simpler. No test, no
>>>> special case, the address is always valid as long as the tcb is valid.
>>> Try implementing it.
>>>
>>> I will post a prototype for my approach within a minute. Its major
>>> implementation advantage is that there is no need to touch any skin,
>>> neither on user nor kernel side, and that there is no need for backward
>>> compatible syscalls.
>>>
>>> Another advantage of my approach is that it does not touch the fast
>>> paths of mutex handling (before deregistration) - well, at lest almost
>>> for non-TLS, but absolutely not for TLS.
>> Do not forget the kernel-space part which detects whether we are using
>> the older or newer user-space.
>
> Not required (famous last words).
>
> The only bit that should be missing in my RFC is the exit() trap,
> probably a two-liner. Will look into this soon.
We discussed about it yesterday. The exit() trap does not guarantee that
the system will be working. So the warning is required.
--
Gilles.
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