Thread stack size issue related to glibc TLS bug

[Florian Weimer]

* Jiangli Zhou:

> Hi Florian,
>
> On Fri, May 24, 2019 at 2:46 AM Florian Weimer <fweimer at redhat.com> wrote:
>>
>> * Jiangli Zhou:
>>
>> > [3] change: http://cr.openjdk.java.net/~jiangli/tls_size/webrev/
>> > (contributed by Jeremy Manson)
>>
>> _dl_get_tls_static_info is an internal symbol (it carries a
>> GLIBC_PRIVATE symbol version).  Its implementation can change at any
>> time.  Please do not do this.
>
> Point taken. Thanks.
>
> It appears that asan is already carrying the same type of fix:
>
> https://github.com/gcc-mirror/gcc/blob/master/libsanitizer/sanitizer_common/sanitizer_linux_libcdep.cc
>
> As the issue has not been able to be addressed in the glibc layer, all
> the others have to workaround it (and possibly by using the glibc
> private APIs, _dl_get_tls_static_info or __pthread_get_minstack). That
> would cause more dependencies on the private APIs.

_dl_get_tls_static_info changed ABI on i386 in glibc 2.27 already, so
you really don't want to use that (in case someone backported that
cleanup to an earlier version, although that might be bit unlikely).

The sanitizers are special and have a much shorter shelf life than the
OpenJDK binaries.

> One alternative (besides fixing glibc) may be making
> _dl_get_tls_static_info public. Would that be possible?

For __pthread_get_minstack, I can add a comment to the glibc sources
that if the ABI changes (or if TLS allocations are no longer considered
part of the stack), we should rename the function.  Then, as long as you
use a weak reference or dlsym (the latter is preferable for the sack of
RPM-based distributions which require special steps to reference
GLIBC_PRIVATE symbols directly), old binaries would keep working if we
make further changes.

Since _dl_get_tls_static_info already changed ABI once, I really can't
recommend its use.  Especially if you can work with
__pthread_get_minstack instead.

The value of PTHREAD_STACK_MIN is rather problematic on x86-64 (for the
reasons I explained earlier), but it's not likely we are going to change
the value of the constant any time soon.  It's more likely that we are
going to work around the AVX-512 register file issue by providing
exactly four usable stack pages with PTHREAD_STACK_MIN, and not less
than two as we did until recently.  So you can assume that it's indeed
possible to use PTHREAD_STACK_MIN and sysconf (_SC_PAGESIZE) to compute
the static TLS area size.

Thanks,
Florian