[PATCH] Bug #20116: Clarify barrier-like and mutex-like behaviours of PD->lock.

[Florian Weimer]

On 03/14/2017 02:07 AM, Carlos O'Donell wrote:
> On 02/14/2017 06:32 AM, Florian Weimer wrote:
>> On 02/13/2017 07:49 PM, Florian Weimer wrote:
>>> On 02/13/2017 02:29 PM, Carlos O'Donell wrote:
>>>> +   It is important to point out that PD->lock is being used as a POSIX
>>>> +   barrier and a POSIX mutex.  The lock is taken in the parent to force
>>>> +   the child to wait, and then the child releases the lock, in effect a
>>>> +   barrier.  However, this barrier-like effect is used only for
>>>> +   synchronizing the parent and child.  After startup the lock is used
>>>> +   like a mutex to create a critical region during which a single owner
>>>> +   modifies the thread parameters.
>>>
>>> I had missed that the lock was reused for the scheduler parameter.
>>>
>>> But the current code still does not make sense to me.  Why do we need to
>>> keep a copy of the scheduler parameters at all?  Is this just a cache to
>>> improve performance, similar to what we used to do for the PID?
>>
>> The cache is used in the implementation of PTHREAD_PRIO_PROTECT mutexes.  There are data races:
>>
>>   https://sourceware.org/bugzilla/show_bug.cgi?id=21160
>>
>> I expect that the use of ->lock to protect these members will go away eventually.
>
> Yes, it's used in tpp.
>
> Given your current understand is the above additional text sufficient
> to clarify the situation?

Yes, I'm mostly fine with it.  I wouldn't call it a “POSIX barrier” and 
“POSIX mutex” though, it clearly is not.

Maybe use “as a barrier (acquired and released by different threads and 
as a mutex (acquired and released by the same thread, providing mutual 
exclusion)”.

Thanks,
Florian