Re: [RFC 5/5] RISC-V: Expand PTHREAD_STACK_MIN to support RVV environment

[Vincent Chen <vincent.chen@sifive.com>]

On Wed, Sep 15, 2021 at 10:31 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Sep 15, 2021 at 3:42 AM Florian Weimer via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > * Joseph Myers:
> >
> > > On Mon, 13 Sep 2021, Vincent Chen wrote:
> > >
> > >> In order to support all pthread operations in the RVV environment, here
> > >> PTHREAD_STACK_MIN is set to 4 times GLRO(dl_minsigstacksize), and the
> > >> default PTHREAD_STACK_MIN is expanded to 20K bytes.
> > >
> > > A change to PTHREAD_STACK_MIN has been considered an ABI change in the
> > > past, requiring new symbol versions for pthread_attr_setstack and
> > > pthread_attr_setstacksize to ensure that binaries built with the old
> > > PTHREAD_STACK_MIN definition continue to work rather than failing because
> > > the old size is too small.  You may need symbol versioning updates for
> > > those functions in RISC-V if you make such a change.  (All the existing
> > > versioning support for this in architecture-independent files assumes the
> > > change in value was done before libpthread was merged into libc, so there
> > > will be some extra work involved in being the first architecture to
> > > increase PTHREAD_STACK_MIN after that merge.)
> >
Hi Joseph,
I understood. I will add symbol versioning to pthread_attr_setstack
and pthread_attr_setstacksize functions if I decide to change the
PTHREAD_STACK_MIN definition. Thank you.

> > Instead it may make sense to leave PTHREAD_STACK_MIN as is and switch to
> > the dynamic version (same for the SIGSTKSZ constants).
> >
>
> I don't know what problem RISC-V ran into.   It should be fixed with:
>

Hi Florian and H.J. Lu,
The dynamic version works for RISC-V. However, I am afraid that some
existing programs, such as nptl/tst-minstack-cancel, do not define
_DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE. In this case, the original
PTHREAD_STACK_MIN definition is too small to support V extension.
Therefore, I finally decided to extend PTHREAD_STACK_MIN to 20K for
normal use cases.

> commit 5d98a7dae955bafa6740c26eaba9c86060ae0344
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Mon Jun 21 12:42:56 2021 -0700
>
>     Define PTHREAD_STACK_MIN to sysconf(_SC_THREAD_STACK_MIN)
>
>     The constant PTHREAD_STACK_MIN may be too small for some processors.
>     Rename _SC_SIGSTKSZ_SOURCE to _DYNAMIC_STACK_SIZE_SOURCE.  When
>     _DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined, define
>     PTHREAD_STACK_MIN to sysconf(_SC_THREAD_STACK_MIN) which is changed
>     to MIN (PTHREAD_STACK_MIN, sysconf(_SC_MINSIGSTKSZ)).
>
>     Consolidate <bits/local_lim.h> with <bits/pthread_stack_min.h> to
>     provide a constant target specific PTHREAD_STACK_MIN value.
>
>     Reviewed-by: Carlos O'Donell <carlos@redhat.com>
>
For this patch, I really appreciate H.J. Lu for providing this feature
in glibc. It's really helpful. But, I have a little question. If
possible, could H.J. Lu help me clarify it?

In __get_pthread_stack_min(), when GLRO(dl_minsigstacksize) !=0 and
GLRO(dl_minsigstacksize) > PTHREAD_STACK_MIN, the pthread_stack_min
will be set to GLRO(dl_minsigstacksize). However, If my understanding
is correct, the size of GLRO(dl_minsigstacksize) approximately equals
the size of the signal context. In this case, the remaining free stack
seems not enough for GCC to execute unwind if this pthread is
terminated by pthread_cancel, such as the case in
tst-minstack-cancel.c. Therefore, my question is that the
PTHREAD_STACK_MIN does not need to reserve the space for GCC to
execute unwind? Thank you in advance.

Regards,
Vincent