NaCl: Make thread exit wake pthread_join.

1 files changed, 47 insertions, 2 deletions

diff --git a/sysdeps/nacl/exit-thread.h b/sysdeps/nacl/exit-thread.h
index a08a5b1d5c..c809405cf1 100644
--- a/sysdeps/nacl/exit-thread.h
+++ b/sysdeps/nacl/exit-thread.h
@@ -16,8 +16,11 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#include <stddef.h>
+#include <assert.h>
+#include <atomic.h>
+#include <lowlevellock.h>
 #include <nacl-interfaces.h>
+#include <nptl/pthreadP.h>
 
 /* This causes the current thread to exit, without affecting other
    threads in the process if there are any.  If there are no other
@@ -26,7 +29,49 @@
 static inline void __attribute__ ((noreturn, always_inline, unused))
 __exit_thread (void)
 {
-  __nacl_irt_thread.thread_exit (NULL);
+  struct pthread *pd = THREAD_SELF;
+
+  /* The generic logic for pthread_join and stack/descriptor reuse is
+     based on the Linux kernel feature that will clear and futex-wake
+     a designated address as a final part of thread teardown.  Correct
+     synchronization relies on the fact that these happen only after
+     there is no possibility of user code touching or examining the
+     late thread's stack.
+
+     The NaCl system interface implements half of this: it clears a
+     word after the thread's user stack is safely dead, but it does
+     not futex-wake the location.  So, some shenanigans are required.
+     We change and futex-wake the location here, so as to wake up any
+     blocked pthread_join (i.e. lll_wait_tid) or pthread_timedjoin_np
+     (i.e. lll_timedwait_tid).  However, that's before we have safely
+     vacated the stack.  So instead of clearing the location, we set
+     it to a special magic value, NACL_EXITING_TID.  This counts as a
+     "live thread" value for all the generic logic, but is recognized
+     specially in lll_wait_tid and lll_timedwait_tid (lowlevellock.h).
+     Once it has this value, lll_wait_tid will busy-wait for the
+     location to be cleared to zero by the NaCl system code.  Only then
+     is the stack actually safe to reuse.  */
+
+  if (!IS_DETACHED (pd))
+    {
+      /* The magic value must not be one that could ever be a valid
+	 TID value.  See pthread-pids.h about the low bit.  */
+      assert (NACL_EXITING_TID & 1);
+
+      /* The magic value must not be one that has the "free" flag
+	 (i.e. sign bit) set.  If that bit is set, then the
+	 descriptor could be reused for a new thread.  */
+      assert (NACL_EXITING_TID > 0);
+
+      atomic_store_relaxed (&pd->tid, NACL_EXITING_TID);
+      lll_futex_wake (&pd->tid, 1, LLL_PRIVATE);
+    }
+
+  /* This clears PD->tid some time after the thread stack can never
+     be touched again.  Unfortunately, it does not also do a
+     futex-wake at that time (as Linux does via CLONE_CHILD_CLEARTID
+     and set_tid_address).  So lll_wait_tid does some busy-waiting.  */
+  __nacl_irt_thread.thread_exit (&pd->tid);
 
   /* That never returns unless something is severely and unrecoverably wrong.
      If it ever does, try to make sure we crash.  */