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Re: New 'PL_lc_numeric_mutex' warnings when building with clang

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From:
Karl Williamson
Date:
February 26, 2018 01:25
Subject:
Re: New 'PL_lc_numeric_mutex' warnings when building with clang
Message ID:
619d836b-c60f-792b-17cc-e356a0aee5b5@khwilliamson.com
On 02/25/2018 06:13 PM, James E Keenan wrote:
> On 02/25/2018 12:13 PM, Karl Williamson wrote:
>> On 02/25/2018 08:12 AM, James E Keenan wrote:
>>> When building perl as I typically do on FreeBSD-11.0 (clang, 
>>> threaded), I have begun to experience a large number of 'make' 
>>> warnings not previously seen.  Gzipped build log attached, but here 
>>> is a taste:
>>
>> I had seen these and researched them, but in order to meet the 
>> deadline, I committed anyway, as they are harmless.  I'm sorry that I 
>> didn't communicate this issue, so that you spent time on it.  We need 
>> to find a way to silence these warnings, and I presume that there is.  
>> I looked them up, and in some documentation about it somewhere, it 
>> admits that this is bogus; that their analysis routine that's 
>> generating these doesn't get this situation right.
>>
>>>
>>> #####
>>> dump.c:497:9: warning: mutex 'PL_lc_numeric_mutex' is not held on 
>>> every path through here [-Wthread-safety-analysis]
>>>          STORE_LC_NUMERIC_SET_STANDARD();
>>>          ^
>>> ./perl.h:5950:13: note: expanded from macro 
>>> 'STORE_LC_NUMERIC_SET_STANDARD'
>>>              LC_NUMERIC_LOCK(_NOT_IN_NUMERIC_STANDARD);     \
>>>              ^
>>> ./perl.h:5593:9: note: expanded from macro 'LC_NUMERIC_LOCK'
>>>          STMT_START {     \
>>>          ^
>>> ./perl.h:530:29: note: expanded from macro 'STMT_START'
>>> #   define STMT_START   (void)( /* gcc supports "({ STATEMENTS; })" */
>>>                                ^
>>> dump.c:497:9: note: mutex acquired here
>>> ./perl.h:5950:13: note: expanded from macro 
>>> 'STORE_LC_NUMERIC_SET_STANDARD'
>>>              LC_NUMERIC_LOCK(_NOT_IN_NUMERIC_STANDARD);     \
>>>              ^
>>> ./perl.h:5595:17: note: expanded from macro 'LC_NUMERIC_LOCK'
>>>                  MUTEX_LOCK(&PL_lc_numeric_mutex);     \
>>>                  ^
>>> ./thread.h:216:14: note: expanded from macro 'MUTEX_LOCK'
>>>          if ((_eC_ = 
>>> perl_pthread_mutex_lock((m))))                      \
>>>                      ^
>>> ./thread.h:206:40: note: expanded from macro 'perl_pthread_mutex_lock'
>>> #    define perl_pthread_mutex_lock(m) perl_tsa_mutex_lock(m)
>>>                                         ^
>>> dump.c:499:9: warning: releasing mutex 'PL_lc_numeric_mutex' that was 
>>> not held [-Wthread-safety-analysis]
>>>          RESTORE_LC_NUMERIC();
>>>          ^
>>> ./perl.h:5923:13: note: expanded from macro 'RESTORE_LC_NUMERIC'
>>>              LC_NUMERIC_UNLOCK;     \
>>>              ^
>>> ./perl.h:5617:17: note: expanded from macro 'LC_NUMERIC_UNLOCK'
>>>                  MUTEX_UNLOCK(&PL_lc_numeric_mutex);     \
>>>                  ^
>>> ./thread.h:224:14: note: expanded from macro 'MUTEX_UNLOCK'
>>>          if ((_eC_ = 
>>> perl_pthread_mutex_unlock((m))))                    \
>>>                      ^
>>> ./thread.h:207:42: note: expanded from macro 'perl_pthread_mutex_unlock'
>>> #    define perl_pthread_mutex_unlock(m) perl_tsa_mutex_unlock(m)
>>>                                           ^
>>> #####
>>>
>>> The first smoke test where I logged these warnings was at this commit:
>>>
>>> #####
>>> $ gitshowf 2a05854a1ad5472d00de22f8fcc284cdc8fd1503
>>> commit 2a05854a1ad5472d00de22f8fcc284cdc8fd1503
>>> Merge: 6de18f4 957ac25
>>> Author:     Father Chrysostomos <sprout@cpan.org>
>>> AuthorDate: Sun Feb 18 16:41:09 2018 -0800
>>> Commit:     Father Chrysostomos <sprout@cpan.org>
>>> CommitDate: Sun Feb 18 16:41:09 2018 -0800
>>>
>>>      [Merge] ‘Nonelem’ scalars
>>>
>>> #####
>>>
>>> However, based on 'git blame perl.h', I suspect that the warnings 
>>> were first introduced here:
>>>
>>> #####
>>> commit 49d7d366441813aa7301b1a437e302372f04f31a
>>> Author:     Karl Williamson <khw@cpan.org>
>>> AuthorDate: Tue Feb 13 13:12:50 2018 -0700
>>> Commit:     Karl Williamson <khw@cpan.org>
>>> CommitDate: Sun Feb 18 15:44:23 2018 -0700
>>>
>>>      Latch LC_NUMERIC during critical sections
>>> #####
>>>
>>> Karl, can you take a look?
>>>
>>> Thank you very much.
>>> Jim Keenan
> 
> One thing I find curious/annoying is that I do not get these warnings 
> when I build with clang (and threaded) on Linux or FreeBSD-10.3 -- only 
> on FreeBSD-11.0.
> 
> Carlos Guevara:  Are you getting warnings like these in your FreeBSD-12 
> smokers?
> 
> Thank you very much.
> Jim Keenan
> 

I believe this is a relatively new clang feature.

These mutexes are not needed unless there is a race condition with 
changing locales.  Therefore they aren't used and there will not be 
warnings on unthreaded builds, or on builds using thread-safe locale 
operations.  Linux has thread-safe builds.  Is the 10.3 clang the same 
as 11.0.  Carlos won't get them on 12, because it is thread-safe.

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