Re: [PATCH] Remove cached PID/TID in clone

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 09/11/2016 10:18, Florian Weimer wrote:
> On 11/08/2016 08:58 PM, Adhemerval Zanella wrote:
> 
>> The tid fields is basically used internally on pthread implementations
>> (including getpid) and since correct usage means thread *must* be
>> created using pthread_create we are sure the tid field will be
>> correctly set due 'set_tid_address' from __pthread_initialize_pids.
> 
> Thanks for the explanation.
> 
> I really think we should document the clone system call wrapper and spell out these requirements, but that's a separate matter.
> 

Right, I think we can update documentation after patch is upstream.

>>> > Please rename to “pid_unused” or something like that, to make sure it's no longer referenced.
>> I renamed it on my local branch and I also updated the change spot
>> that it incur:
>>
>> diff --git a/nptl_db/structs.def b/nptl_db/structs.def
>> index a9b621b..1cb6a46 100644
>> --- a/nptl_db/structs.def
>> +++ b/nptl_db/structs.def
>> @@ -48,7 +48,6 @@ DB_STRUCT (pthread)
>>  DB_STRUCT_FIELD (pthread, list)
>>  DB_STRUCT_FIELD (pthread, report_events)
>>  DB_STRUCT_FIELD (pthread, tid)
>> -DB_STRUCT_FIELD (pthread, pid)
>>  DB_STRUCT_FIELD (pthread, start_routine)
>>  DB_STRUCT_FIELD (pthread, cancelhandling)
>>  DB_STRUCT_FIELD (pthread, schedpolicy)
> 
> Have you tested that thread debugging still works after these changes (at least on one architecture)?
> 

I just checked with binutils gdb.threads testcase and saw no
regressions.

>> The patch also removes the TID caching in clone. My understanding for
>> such semantic is try provide some pthread usage after a user program
>> issue clone directly (as done by thread creation with CLONE_PARENT_SETTID
>> and pthread tid member).  However, as stated before in multiple threads,
> 
> “discussion threads”? ☹

Ack, I changed it locally.

> 
>> GLIBC provides clone syscalls without further supporting all this
>> semantics. It means that, although GLIBC currently tries a better effort,
>> since it does not make any more guarantees, specially for newer and newer
>> clone flags.
> 
> I don't quite understand the above part.
> 
>>     * sysdeps/unix/sysv/linux/getpid.c: Likewise.
> 
> This needs updating (file was removed).

Ack.

> 
> I do not have further comments, but I have not reviewed the assembler language implementations (only i386/x86_64).  I support the removal of PID caching, though.
> 

I also did a full check on aarch64, powerpc64le, and armhf.  I also did 
some basic tests (basically the posix and nptl one involving clone/fork)
on a simulated sparc{64} and mips{64} machine to check if I missed 
something in clone/vfork assembly changes.