[ltt-dev] [UST PATCH] libust: Fix multiple fd close during fork

[Nils Carlson]

On Wed, 23 Feb 2011, Mathieu Desnoyers wrote:

> * Nils Carlson (nils.carlson at ericsson.com) wrote:
>>
>>
>> On Wed, 23 Feb 2011, Mathieu Desnoyers wrote:
>>
>>> * Nils Carlson (nils.carlson at ericsson.com) wrote:
>>>> On Wed, 23 Feb 2011, Mathieu Desnoyers wrote:
>>>>
>>>>> * Nils Carlson (nils.carlson at ericsson.com) wrote:
>>>>>> Yep. fixed. merged.
>>>>>>
>>>>>>>>> +    /*
>>>>>>>>> +     * FIXME: This could be prettier, we loop over the list twice and
>>>>>>>>> +     * following good locking practice should lock around the loop
>>>>>>>>> +     */
>>>>>>>>> +    cds_list_for_each_entry_safe(trace, trace_tmp, &ltt_traces.head,
>>>>>>>>> list) {
>>>>>>>>> +        ltt_trace_destroy(trace->trace_name, 1);
>>>>>>>>> +    }
>>>>>
>>>>> So what' up with these missing locks ?
>>>>>
>>>>
>>>> They are actually taken by each ltt_trace_destroy. Also, this function is
>>>> run as part of the ust_fork so locking in the child is a non-issue at
>>>> this point. Its mostly an aesthetic thing that we should be consistent in
>>>> dealing with locks.
>>>
>>> Most of the ust_fork code should actually be protected by a mutex to
>>> deal with the fact that we might have a concurrent libust thread running
>>> in the parent at the exact point we do the fork, thus letting the child
>>> in a state where locks are taken, and waits on the locks forever,
>>> because the libust thread is not present in the child. The concurrent
>>> libust thread in the parent should be kept quiescent while we do the
>>> fork.
>>>
>>
>> ust_fork is only called by the child after the fork. The function name is
>> a bit misleading.
>>
>>> I don't think this is handled at the moment, and can leave us with
>>> various of the other locks in a "held" state when we run in the child.
>>
>> As far as locking is concerned we're completely safe today, as we are
>> completely serialised. If we ever decide on multiple control threads or
>> some such we will probably have to recheck all the locking.
>
> Let's look at:
>
> ustfork.c:
>
> Parent                           Child
>
> fork()
>  ust_before_fork()
>    - disable signals
>  plibc_func(); (real fork)
>  ust_after_fork_parent()
>    - restore signals
>                                 ust_after_fork_child()
>                                   - ust_fork()
>                                       ltt_unlock_traces(); <- bug (never locked ?)
>                                   - reenable signals
>
> So if we have a libust listener thread running in the parent in parallel
> with the fork(), we possibly leave many locks in "held" state when the
> child starts, and we don't want that, right ? It looks like a can of
> worms ready to be opened. Ideally, we should exclude the libust listener
> thread by taking a mutex across the fork in ust_before_fork (after
> disabling signals) and releasing it in both ust_after_fork_parent and
> ust_after_fork_child.

I can have a look tomorrow. We don't have all that many locks though. Most 
of my focus has been on not causing deadlocks. I've sort of ducked the 
fork issue this far as their are some things to consider.

I think your solution of a cross-fork mutex is a good one. This would 
allow us to avoid the current pitfalls with memory allocation (currently 
if you fork before saving a reference to the allocated memory somewhere it 
will be lost resulting in a leak.)

Yepp. Think thats a good solution. Will think about it tonight. :-)

/Nils


> Thoughts ?
>
> Mathieu
>
>
>>
>> /Nils
>>
>>
>>> Thoughts ?
>>>
>>> Mathieu
>>>
>>>>
>>>> /Nils
>>>>
>>>>> Mathieu
>>>>>
>>>>>>>>>
>>>>>>>>>     /* Clean up the listener socket and epoll, keeping the scoket
>>>>>>>>> file */
>>>>>>>>>     ustcomm_del_named_sock(listen_sock, 1);
>>>>>>>>> --
>>>>>>>>> 1.7.1
>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> ltt-dev mailing list
>>>>>> ltt-dev at lists.casi.polymtl.ca
>>>>>> http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
>>>>>>
>>>>>
>>>>> --
>>>>> Mathieu Desnoyers
>>>>> Operating System Efficiency R&D Consultant
>>>>> EfficiOS Inc.
>>>>> http://www.efficios.com
>>>>>
>>>>
>>>
>>> --
>>> Mathieu Desnoyers
>>> Operating System Efficiency R&D Consultant
>>> EfficiOS Inc.
>>> http://www.efficios.com
>>>
>>
>
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>