Fix bug in pthread_join, pthread_exit, pthread_detach

pthread_no_op_detach_after_join test from bionic-unit-tests hangs
on x86 emulator. There is a race in the pthread_join, pthread_exit,
pthread_detach functions:
- pthread_join waits for the non-detached thread
- pthread_detach sets the detached flag on that thread
- the thread executes pthread_exit which just kills the now-detached
thread, without sending the join notification.

This patch improves the test so it fails on ARM too, and modifies
pthread_detach to behave more like glibc, not setting the detach state if
called on a thread that's already being joined (but not returning an error).

Change-Id: I87dc688221ce979ef5178753dd63d01ac0b108e6
Signed-off-by: Sergey Melnikov <sergey.melnikov@intel.com>

libc/bionic/pthread.c

@@ -582,7 +582,7 @@ void pthread_exit(void * retval)
     pthread_key_clean_all();
 
     // if the thread is detached, destroy the pthread_internal_t
-    // otherwise, keep it in memory and signal any joiners
+    // otherwise, keep it in memory and signal any joiners.
     if (thread->attr.flags & PTHREAD_ATTR_FLAG_DETACHED) {
         _pthread_internal_remove(thread);
         _pthread_internal_free(thread);
@@ -668,8 +668,9 @@ FoundIt:
         pthread_cond_wait( &thread->join_cond, &gThreadListLock );
         count = --thread->join_count;
     }
-    if (ret_val)
+    if (ret_val) {
         *ret_val = thread->return_value;
+    }
 
     /* remove thread descriptor when we're the last joiner or when the
      * thread was already a zombie.
@@ -686,28 +687,30 @@ int  pthread_detach( pthread_t  thid )
 {
     pthread_internal_t*  thread;
     int                  result = 0;
-    int                  flags;
 
     pthread_mutex_lock(&gThreadListLock);
-    for (thread = gThreadList; thread != NULL; thread = thread->next)
-        if (thread == (pthread_internal_t*)thid)
+    for (thread = gThreadList; thread != NULL; thread = thread->next) {
+        if (thread == (pthread_internal_t*)thid) {
             goto FoundIt;
+        }
+    }
 
     result = ESRCH;
     goto Exit;
 
 FoundIt:
-    do {
-        flags = thread->attr.flags;
-
-        if ( flags & PTHREAD_ATTR_FLAG_DETACHED ) {
-            /* thread is not joinable ! */
-            result = EINVAL;
-            goto Exit;
-        }
+    if (thread->attr.flags & PTHREAD_ATTR_FLAG_DETACHED) {
+        result = EINVAL; // Already detached.
+        goto Exit;
     }
-    while ( __bionic_cmpxchg( flags, flags | PTHREAD_ATTR_FLAG_DETACHED,
-                              (volatile int*)&thread->attr.flags ) != 0 );
+
+    if (thread->join_count > 0) {
+        result = 0; // Already being joined; silently do nothing, like glibc.
+        goto Exit;
+    }
+
+    thread->attr.flags |= PTHREAD_ATTR_FLAG_DETACHED;
+
 Exit:
     pthread_mutex_unlock(&gThreadListLock);
     return result;

tests/pthread_test.cpp

@@ -37,10 +37,26 @@ static void* SleepFn(void* arg) {
   return NULL;
 }
 
+static void* SpinFn(void* arg) {
+  volatile bool* b = reinterpret_cast<volatile bool*>(arg);
+  while (!*b) {
+  }
+  return NULL;
+}
+
 static void* JoinFn(void* arg) {
   return reinterpret_cast<void*>(pthread_join(reinterpret_cast<pthread_t>(arg), NULL));
 }
 
+static void AssertDetached(pthread_t t, bool is_detached) {
+  pthread_attr_t attr;
+  ASSERT_EQ(0, pthread_getattr_np(t, &attr));
+  int detach_state;
+  ASSERT_EQ(0, pthread_attr_getdetachstate(&attr, &detach_state));
+  pthread_attr_destroy(&attr);
+  ASSERT_EQ(is_detached, (detach_state == PTHREAD_CREATE_DETACHED));
+}
+
 TEST(pthread, pthread_create) {
   void* expected_result = reinterpret_cast<void*>(123);
   // Can we create a thread?
@@ -58,6 +74,7 @@ TEST(pthread, pthread_no_join_after_detach) {
 
   // After a pthread_detach...
   ASSERT_EQ(0, pthread_detach(t1));
+  AssertDetached(t1, true);
 
   // ...pthread_join should fail.
   void* result;
@@ -65,20 +82,27 @@ TEST(pthread, pthread_no_join_after_detach) {
 }
 
 TEST(pthread, pthread_no_op_detach_after_join) {
+  bool done = false;
+
   pthread_t t1;
-  ASSERT_EQ(0, pthread_create(&t1, NULL, SleepFn, reinterpret_cast<void*>(1)));
+  ASSERT_EQ(0, pthread_create(&t1, NULL, SpinFn, &done));
 
   // If thread 2 is already waiting to join thread 1...
   pthread_t t2;
   ASSERT_EQ(0, pthread_create(&t2, NULL, JoinFn, reinterpret_cast<void*>(t1)));
 
-  // ...a call to pthread_detach on thread 1 will "succeed"...
-  ASSERT_EQ(0, pthread_detach(t1));
+  sleep(1); // (Give t2 a chance to call pthread_join.)
 
-  // ...but the join still goes ahead.
+  // ...a call to pthread_detach on thread 1 will "succeed" (silently fail)...
+  ASSERT_EQ(0, pthread_detach(t1));
+  AssertDetached(t1, false);
+
+  done = true;
+
+  // ...but t2's join on t1 still goes ahead (which we can tell because our join on t2 finishes).
   void* join_result;
   ASSERT_EQ(0, pthread_join(t2, &join_result));
-  ASSERT_EQ(EINVAL, reinterpret_cast<int>(join_result));
+  ASSERT_EQ(0, reinterpret_cast<int>(join_result));
 }
 
 TEST(pthread, pthread_join_self) {