RFC elf: Fix slow tls access after dlopen [BZ #19924] nsz/bug19329-v2

In short: __tls_get_addr checks the global generation counter,
_dl_update_slotinfo updates up to the generation of the accessed
module. If the global generation is newer than geneneration of the
module then __tls_get_addr keeps hitting the slow path that updates
the dtv.

Possible approaches i can see:

1. update to global generation instead of module,
2. check the module generation in the fast path.

This patch is 1.: it needs additional sync (load acquire) so the
slotinfo list is up to date with the observed global generation.

Approach 2. would require walking the slotinfo list at all times.
I don't know how to make that fast with many modules.

Note: in the x86_64 version of dl-tls.c the generation is only loaded
once, since relaxed mo is not faster than acquire mo load.

I have not benchmarked this yet.

1 files changed, 2 insertions, 1 deletions

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index ea3f7a69d0..614463f016 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1224,7 +1224,8 @@ extern void _dl_add_to_slotinfo (struct link_map *l, bool do_add)
 
 /* Update slot information data for at least the generation of the
    module with the given index.  */
-extern struct link_map *_dl_update_slotinfo (unsigned long int req_modid)
+extern struct link_map *_dl_update_slotinfo (unsigned long int req_modid,
+					     size_t gen)
      attribute_hidden;
 
 /* Look up the module's TLS block as for __tls_get_addr,