libthr shared locks

Daniel Eischen deischen at
Thu Dec 24 13:45:41 UTC 2015

On Wed, 23 Dec 2015, John Baldwin wrote:

> On Wednesday, December 23, 2015 10:18:37 PM Konstantin Belousov wrote:
>> On Wed, Dec 23, 2015 at 02:48:56PM -0500, Daniel Eischen wrote:
>>> On Wed, 23 Dec 2015, Konstantin Belousov wrote:
>>>> On Wed, Dec 23, 2015 at 01:27:35PM -0500, Daniel Eischen wrote:
>>>>> On Wed, 23 Dec 2015, Konstantin Belousov wrote:
>>>>> [ ... ]
>>>>>> Would the ABI modified to make the pthread_mutex_t large enough to
>>>>>> hold struct pthread_mutex, the rest of the implementation of the
>>>>>> shared mutex is relatively trivial, if not already done.
>>>>>> Changing this ABI is very hard.  libthr provides the symbol
>>>>>> versioning, which allows to provide compatible shims for the previous
>>>>>> ABI variant.  But since userspace tends to use the pthread objects in
>>>>>> the layouts of the library objects, this causes serious ABI issues
>>>>>> when mixing libraries built against different default versions of
>>>>>> libthr.
>>>>> I think this is only if the libraries (or apps) pass pthread
>>>>> lock types between them, that one has initialized with one ABI
>>>>> but the other library uses another ABI.  We should really
>>>>> exclude this as part of our ABI compatibility.
>>>> Applications commonly embed pthread locks into their objects, including
>>>> the exposed ABI in the third-party libraries.
>>>> struct my_object {
>>>> 	pthread_mutex_t obj_lock;
>>>> 	...
>>>> };
>>>> Changing the size of the pthread locks causes uncontrolled breakage of
>>>> the ABI for significant number of third-party code.
>>> If the application creates the object itself or allocates storage
>>> for it, basically, if it isn't opaque, yes.  But we can bump port
>>> revisions for affected libraries (probably just searching
>>> /usr/local/include/... for pthread_mutex, pthread_cond, etc
>>> types to see possible problems.  I did a search for the installed
>>> ports on my system and found a few that might cause problems.
>> This relegates the issue to an attempt to do the full rebuild.  But I
>> do not see how the port bump would fix it, assume that you are updating
>> from the 10.x to 11.x and have the mix of the libraries, some of which
>> were built during the 10.x lifetime but with the bumped ports version.
>> It is not feasible to do a reliable audit of the 24+ Kports.
> As a bit of a devil's advocate, I think the 64-bit ino_t change will in
> fact require this for 11.  I suspect 3rd pary apps embed struct stat in
> various structures as well and that that ABI change will require not
> mixing old and new libraries.
> One other point in favor of Konstantin's approach (IMO) is that keeping
> the structures private prevents having to maintain the ABI of those
> structures in the future.  I'm already keenly aware of how painful a
> problem that can be with our non-opaque FILE (and which we cannot now
> make opaque even though the standard APIs would work fine with an opaque
> object).

We would include extra/spare words in the struct from day 1.
The public struct should just consist of an array of storage
units or similar, so that nothing can be gleaned from the
elements of the struct.  This is what Solaris does (or at least
used to).

Going forward, I think the sync structures just need to be able
to be properly initialized with PTHREAD_FOO_INITIALIZER.  If
10 years out the spare words are not enough, we could still
allocate remaining storage on first use like we do now.


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