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


On Jun  3, 2015, Alexandre Oliva <aoliva@redhat.com> wrote:

> How's this?

Or rather this, that I got after updating the patch file :-)
(the testcase is adjusted to use atomics)


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 without need introduces a significant
performance penalty, and 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, as in the testcase.

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; when tls dtors access TLS variables, using dynamic
access models, from modules that have not yet been resolved to static
or dynamic TLS, or any other explicit or implicit interactions with
the dynamic loader in ways that require its lock to be taken.

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, from Andreas
	Schwab's bug report.
	* nptl/Makefile (tests): Add tst-join7.
	(module-names): Add tst-join7mod.
	($(objpfx)tst-join7): New.  Add deps.
	($(objpfx)tst-join7.out): Likewise.
	($(objpfx)tst-join7mod.so): Likewise.
	(LDFLAGS-tst-join7mod.so): Set soname.
---
 NEWS                |    2 +-
 elf/dl-tls.c        |   63 +++++++++++++++++++++++++++++++--------------------
 nptl/Makefile       |   10 ++++++--
 nptl/tst-join7.c    |   12 ++++++++++
 nptl/tst-join7mod.c |   30 ++++++++++++++++++++++++
 5 files changed, 89 insertions(+), 28 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..8fc210d 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -755,41 +755,54 @@ 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 some static TLS offset, the decision is final, and no lock is
+     required.  Now, if the decision hasn't been made, take the rtld
+     lock, so that an ongoing dlopen gets a chance to complete,
+     possibly assigning the module to static TLS and initializing the
+     corresponding TLS area for all threads, and then retest; if the
+     decision is still pending, force the module to dynamic TLS.
+
+     The risk that the thread accesses an earlier value in that memory
+     location, from before it was recycled into a link map in another
+     thread, is removed by the need for some happens before
+     relationship between the loader that set that up and the TLS
+     access that referenced the module id.  In the presence of such a
+     relationship, the value will be at least as recent as the
+     initialization, and in its absence, calling tls_get_addr with its
+     module id invokes undefined behavior.  */
+  if (__glibc_unlikely (the_map->l_tls_offset == NO_TLS_OFFSET))
     {
       __rtld_lock_lock_recursive (GL(dl_load_lock));
       if (__glibc_likely (the_map->l_tls_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))
-	{
+	the_map->l_tls_offset = FORCED_DYNAMIC_TLS_OFFSET;
+      __rtld_lock_unlock_recursive (GL(dl_load_lock));
+    }
+
+  void *p;
+
+  if (the_map->l_tls_offset != FORCED_DYNAMIC_TLS_OFFSET)
+    {
 #if TLS_TCB_AT_TP
-	  void *p = (char *) THREAD_SELF - the_map->l_tls_offset;
+      p = (char *) THREAD_SELF - the_map->l_tls_offset;
 #elif TLS_DTV_AT_TP
-	  void *p = (char *) THREAD_SELF + the_map->l_tls_offset + TLS_PRE_TCB_SIZE;
+      p = (char *) THREAD_SELF + the_map->l_tls_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;
 
-	  return (char *) p + GET_ADDR_OFFSET;
-	}
-      else
-	__rtld_lock_unlock_recursive (GL(dl_load_lock));
+      dtv[GET_ADDR_MODULE].pointer.is_static = true;
+      dtv[GET_ADDR_MODULE].pointer.val = p;
+    }
+  else
+    {
+      p = allocate_and_init (the_map);
+      assert (!dtv[GET_ADDR_MODULE].pointer.is_static);
+      /* FIXME: this is AS-Unsafe.  We'd have to atomically set val to
+	 p, if it is still unallocated, or release p and set it to
+	 val.  */
+      dtv[GET_ADDR_MODULE].pointer.val = p;
     }
-  void *p = dtv[GET_ADDR_MODULE].pointer.val = allocate_and_init (the_map);
-  assert (!dtv[GET_ADDR_MODULE].pointer.is_static);
 
   return (char *) p + GET_ADDR_OFFSET;
 }
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..9960b76
--- /dev/null
+++ b/nptl/tst-join7mod.c
@@ -0,0 +1,30 @@
+#include <stdio.h>
+#include <pthread.h>
+#include <atomic.h>
+
+static pthread_t th;
+static int running = 1;
+
+static void *
+test_run (void *p)
+{
+  while (atomic_load_relaxed (&running))
+    printf ("XXX test_run\n");
+  printf ("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)
+{
+  atomic_store_relaxed (&running, 0);
+  printf ("thread_join...\n");
+  pthread_join (th, NULL);
+  printf ("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


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