[PR18457] Don't require rtld lock to compute DTV addr for static TLS

[Alexandre Oliva <aoliva at redhat dot com>]

We used to store static TLS addrs in the DTV at module load time, but
this required one thread to modify another thread's DTV.  Now that we
defer the DTV update to the first use in the thread, we should do so
without taking the rtld lock if the module is already assigned to static
TLS.  Taking the lock introduces deadlocks where there weren't any
before.

This patch fixes the deadlock caused by tls_get_addr's unnecessarily
taking the rtld lock to initialize the DTV entry for tls_dtor_list
within __call_dtors_list, which deadlocks with a dlclose of a module
whose finalizer joins with that thread.  The patch does not, however,
attempt to fix other potential sources of similar deadlocks, such as
the explicit rtld locks taken by call_dtors_list, when the dtor list
is not empty; lazy relocation of the reference to tls_dtor_list, when
TLS Descriptors are in use; when tls dtors call functions through the
PLT and lazy relocation needs to be performed, or any such called
functions interact with the dynamic loader in ways that require its
lock to be taken.

Ok to install?


for  ChangeLog

	[PR dynamic-link/18457]
	* elf/dl-tls.c (tls_get_addr_tail): Don't take the rtld lock
	if we already have a final static TLS offset.
	* nptl/tst-join7.c, nptl/tst-join7mod.c: New.
---
 NEWS                |    2 +-
 elf/dl-tls.c        |   38 ++++++++++++++++++++++++++------------
 nptl/Makefile       |   10 ++++++++--
 nptl/tst-join7.c    |   12 ++++++++++++
 nptl/tst-join7mod.c |   29 +++++++++++++++++++++++++++++
 5 files changed, 76 insertions(+), 15 deletions(-)
 create mode 100644 nptl/tst-join7.c
 create mode 100644 nptl/tst-join7mod.c

diff --git a/NEWS b/NEWS
index ed02de0..eac100c 100644
--- a/NEWS
+++ b/NEWS
@@ -19,7 +19,7 @@ Version 2.22
   18047, 18049, 18068, 18080, 18093, 18100, 18104, 18110, 18111, 18116,
   18125, 18128, 18138, 18185, 18196, 18197, 18206, 18210, 18211, 18217,
   18220, 18221, 18234, 18244, 18247, 18287, 18319, 18333, 18346, 18397,
-  18409, 18410, 18412, 18418, 18422, 18434, 18444, 18469.
+  18409, 18410, 18412, 18418, 18422, 18434, 18444, 18457, 18469.
 
 * Cache information can be queried via sysconf() function on s390 e.g. with
   _SC_LEVEL1_ICACHE_SIZE as argument.
diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index 20c7e33..726f1ea 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -755,30 +755,44 @@ tls_get_addr_tail (GET_ADDR_ARGS, dtv_t *dtv, struct link_map *the_map)
       the_map = listp->slotinfo[idx].map;
     }
 
-  /* Make sure that, if a dlopen running in parallel forces the
-     variable into static storage, we'll wait until the address in the
-     static TLS block is set up, and use that.  If we're undecided
-     yet, make sure we make the decision holding the lock as well.  */
-  if (__glibc_unlikely (the_map->l_tls_offset
-			!= FORCED_DYNAMIC_TLS_OFFSET))
+  /* If the TLS block for the map is already assigned to dynamic or to
+     static TLS, avoid the lock.  Be careful to use the same value for
+     both tests; if we reloaded it, the second test might mistake
+     forced dynamic for an offset.  Now, if the decision hasn't been
+     made, take the rtld lock, so that an ongoing dlopen gets a chance
+     to complete, and then retest; if the decision is still pending,
+     force the module to dynamic TLS.  */
+  ptrdiff_t offset = atomic_load_relaxed (&the_map->l_tls_offset);
+  if (__glibc_unlikely (offset != FORCED_DYNAMIC_TLS_OFFSET))
     {
+      if (__glibc_unlikely (offset != NO_TLS_OFFSET))
+	goto static_tls;
       __rtld_lock_lock_recursive (GL(dl_load_lock));
-      if (__glibc_likely (the_map->l_tls_offset == NO_TLS_OFFSET))
+      offset = the_map->l_tls_offset;
+      if (__glibc_likely (offset == NO_TLS_OFFSET))
 	{
 	  the_map->l_tls_offset = FORCED_DYNAMIC_TLS_OFFSET;
 	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
 	}
-      else if (__glibc_likely (the_map->l_tls_offset
-			       != FORCED_DYNAMIC_TLS_OFFSET))
+      else if (__glibc_likely (offset != FORCED_DYNAMIC_TLS_OFFSET))
 	{
+	  /* The decision is made, and it is final.  We use the value
+	     we've already loaded, but we could even load the offset
+	     after releasing the lock, since it won't change.  Should
+	     the module be released while another thread references
+	     one of its TLS variables, that's undefined behavior.  */
+	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
+
+	static_tls:
+	  ;
+
 #if TLS_TCB_AT_TP
-	  void *p = (char *) THREAD_SELF - the_map->l_tls_offset;
+	  void *p = (char *) THREAD_SELF - offset;
 #elif TLS_DTV_AT_TP
-	  void *p = (char *) THREAD_SELF + the_map->l_tls_offset + TLS_PRE_TCB_SIZE;
+	  void *p = (char *) THREAD_SELF + offset + TLS_PRE_TCB_SIZE;
 #else
 # error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
 #endif
-	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
 
 	  dtv[GET_ADDR_MODULE].pointer.is_static = true;
 	  dtv[GET_ADDR_MODULE].pointer.val = p;
diff --git a/nptl/Makefile b/nptl/Makefile
index 3dd2944..4ffd13c 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -242,7 +242,7 @@ tests = tst-typesizes \
 	tst-basic7 \
 	tst-kill1 tst-kill2 tst-kill3 tst-kill4 tst-kill5 tst-kill6 \
 	tst-raise1 \
-	tst-join1 tst-join2 tst-join3 tst-join4 tst-join5 tst-join6 \
+	tst-join1 tst-join2 tst-join3 tst-join4 tst-join5 tst-join6 tst-join7 \
 	tst-detach1 \
 	tst-eintr1 tst-eintr2 tst-eintr3 tst-eintr4 tst-eintr5 \
 	tst-tsd1 tst-tsd2 tst-tsd3 tst-tsd4 tst-tsd5 tst-tsd6 \
@@ -320,7 +320,8 @@ endif
 modules-names = tst-atfork2mod tst-tls3mod tst-tls4moda tst-tls4modb \
 		tst-tls5mod tst-tls5moda tst-tls5modb tst-tls5modc \
 		tst-tls5modd tst-tls5mode tst-tls5modf tst-stack4mod \
-		tst-_res1mod1 tst-_res1mod2 tst-execstack-mod tst-fini1mod
+		tst-_res1mod1 tst-_res1mod2 tst-execstack-mod tst-fini1mod \
+		tst-join7mod
 extra-test-objs += $(addsuffix .os,$(strip $(modules-names))) tst-cleanup4aux.o
 test-extras += $(modules-names) tst-cleanup4aux
 test-modules = $(addprefix $(objpfx),$(addsuffix .so,$(modules-names)))
@@ -525,6 +526,11 @@ $(objpfx)tst-tls6.out: tst-tls6.sh $(objpfx)tst-tls5 \
 	$(evaluate-test)
 endif
 
+$(objpfx)tst-join7: $(libdl) $(shared-thread-library)
+$(objpfx)tst-join7.out: $(objpfx)tst-join7mod.so
+$(objpfx)tst-join7mod.so: $(shared-thread-library)
+LDFLAGS-tst-join7mod.so = -Wl,-soname,tst-join7mod.so
+
 $(objpfx)tst-dlsym1: $(libdl) $(shared-thread-library)
 
 $(objpfx)tst-fini1: $(shared-thread-library) $(objpfx)tst-fini1mod.so
diff --git a/nptl/tst-join7.c b/nptl/tst-join7.c
new file mode 100644
index 0000000..bf6fc76
--- /dev/null
+++ b/nptl/tst-join7.c
@@ -0,0 +1,12 @@
+#include <dlfcn.h>
+
+int
+do_test (void)
+{
+  void *f = dlopen ("tst-join7mod.so", RTLD_NOW | RTLD_GLOBAL);
+  if (f) dlclose (f); else return 1;
+  return 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/nptl/tst-join7mod.c b/nptl/tst-join7mod.c
new file mode 100644
index 0000000..a8c7bc0
--- /dev/null
+++ b/nptl/tst-join7mod.c
@@ -0,0 +1,29 @@
+#include <stdio.h>
+#include <pthread.h>
+
+static pthread_t th;
+static int running = 1;
+
+static void *
+test_run (void *p)
+{
+  while (running)
+    fprintf (stderr, "XXX test_run\n");
+  fprintf (stderr, "XXX test_run FINISHED\n");
+  return NULL;
+}
+
+static void __attribute__ ((constructor))
+do_init (void)
+{
+  pthread_create (&th, NULL, test_run, NULL);
+}
+
+static void __attribute__ ((destructor))
+do_end (void)
+{
+  running = 0;
+  fprintf (stderr, "thread_join...\n");
+  pthread_join (th, NULL);
+  fprintf (stderr, "thread_join DONE\n");
+}


-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer