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

[nils.carlson@ericsson.com (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:
>>>> 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.

/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
>>>>>>>
>>>>>>>
>>>>>
>>>>>
>>>>
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>>>>
>>>
>>> --
>>> 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
>