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

[Florian Weimer <fweimer@redhat.com>]

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.

>> > 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)?

> 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”? ☹

> 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).

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.

Thanks,
Florian