lockdep: fix races with concurrent lockdep teardown

If the cct is unregistered while other threads are flogging mutexes,
then we can hit all sorts of bugs. Ensure that we handle that
situation sanely, by checking that g_lockdep is still set after
we take the lockdep_mutex.

Also, remove an assertion from lockdep_unregister, and just turn it into
an immediate return. It's possible to have a call to
lockdep_unregister_ceph_context, and then a call to
lockdep_register_ceph_context while a mutex is being held by another
task.

In that case, it's possible the lock does not exist in the map
when we go to unregister it. That's not a bug though, just a natural
consequence of that series of actions.

Tracker: http://tracker.ceph.com/issues/20988
Signed-off-by: Jeff Layton <jlayton@redhat.com>

src/common/lockdep.cc

@@ -113,6 +113,8 @@ void lockdep_unregister_ceph_context(CephContext *cct)
 int lockdep_dump_locks()
 {
   pthread_mutex_lock(&lockdep_mutex);
+  if (!g_lockdep)
+    goto out;
 
   for (ceph::unordered_map<pthread_t, map<int,BackTrace*> >::iterator p = held.begin();
        p != held.end();
@@ -127,7 +129,7 @@ int lockdep_dump_locks()
       *_dout << dendl;
     }
   }
-
+out:
   pthread_mutex_unlock(&lockdep_mutex);
   return 0;
 }
@@ -165,8 +167,10 @@ int lockdep_get_free_id(void)
 
 static int _lockdep_register(const char *name)
 {
-  int id;
+  int id = -1;
 
+  if (!g_lockdep)
+    return id;
   ceph::unordered_map<std::string, int>::iterator p = lock_ids.find(name);
   if (p == lock_ids.end()) {
     id = lockdep_get_free_id();
@@ -211,9 +215,16 @@ void lockdep_unregister(int id)
   }
 
   pthread_mutex_lock(&lockdep_mutex);
+  if (!g_lockdep) {
+    pthread_mutex_unlock(&lockdep_mutex);
+    return;
+  }
 
   map<int, std::string>::iterator p = lock_names.find(id);
-  assert(p != lock_names.end());
+  if (p == lock_names.end()) {
+    pthread_mutex_unlock(&lockdep_mutex);
+    return;
+  }
 
   int &refs = lock_refs[id];
   if (--refs == 0) {
@@ -279,8 +290,14 @@ int lockdep_will_lock(const char *name, int id, bool force_backtrace)
   pthread_t p = pthread_self();
 
   pthread_mutex_lock(&lockdep_mutex);
+  if (!g_lockdep) {
+    pthread_mutex_unlock(&lockdep_mutex);
+    return id;
+  }
+
   if (id < 0)
     id = _lockdep_register(name);
+
   lockdep_dout(20) << "_will_lock " << name << " (" << id << ")" << dendl;
 
   // check dependency graph
@@ -343,7 +360,6 @@ int lockdep_will_lock(const char *name, int id, bool force_backtrace)
       }
     }
   }
-
   pthread_mutex_unlock(&lockdep_mutex);
   return id;
 }
@@ -353,6 +369,8 @@ int lockdep_locked(const char *name, int id, bool force_backtrace)
   pthread_t p = pthread_self();
 
   pthread_mutex_lock(&lockdep_mutex);
+  if (!g_lockdep)
+    goto out;
   if (id < 0)
     id = _lockdep_register(name);
 
@@ -361,6 +379,7 @@ int lockdep_locked(const char *name, int id, bool force_backtrace)
     held[p][id] = new BackTrace(BACKTRACE_SKIP);
   else
     held[p][id] = 0;
+out:
   pthread_mutex_unlock(&lockdep_mutex);
   return id;
 }
@@ -376,6 +395,8 @@ int lockdep_will_unlock(const char *name, int id)
   }
 
   pthread_mutex_lock(&lockdep_mutex);
+  if (!g_lockdep)
+    goto out;
   lockdep_dout(20) << "_will_unlock " << name << dendl;
 
   // don't assert.. lockdep may be enabled at any point in time
@@ -384,6 +405,7 @@ int lockdep_will_unlock(const char *name, int id)
 
   delete held[p][id];
   held[p].erase(id);
+out:
   pthread_mutex_unlock(&lockdep_mutex);
   return id;
 }