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, 1 insertions, 1 deletions

diff --git a/elf/dl-close.c b/elf/dl-close.c
index 9f31532f41..45f8a7fe31 100644
--- a/elf/dl-close.c
+++ b/elf/dl-close.c
@@ -780,7 +780,7 @@ _dl_close_worker (struct link_map *map, bool force)
       if (__glibc_unlikely (newgen == 0))
 	_dl_fatal_printf ("TLS generation counter wrapped!  Please report as described in "REPORT_BUGS_TO".\n");
       /* Can be read concurrently.  */
-      atomic_store_relaxed (&GL(dl_tls_generation), newgen);
+      atomic_store_release (&GL(dl_tls_generation), newgen);
 
       if (tls_free_end == GL(dl_tls_static_used))
 	GL(dl_tls_static_used) = tls_free_start;