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Re: New 'PL_lc_numeric_mutex' warnings when building with clang

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From:
James E Keenan
Date:
February 26, 2018 02:49
Subject:
Re: New 'PL_lc_numeric_mutex' warnings when building with clang
Message ID:
efb0d844-33ee-9394-582f-4fc3c28fedf6@pobox.com
On 02/25/2018 08:24 PM, Karl Williamson wrote:
> 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. 

fbsd 10.3: clang 3.4.1
fbsd 11.0: clang 3.8.0

  Carlos won't get them on 12, because it is thread-safe.

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