Lockdep treats down_write_trylock like regular down_write

This causes constructions like

down_write(&mm1->mmap_sem);
if (down_write_trylock(&mm2->mmap_sem)) {
       ...
       up_write(&mm2->mmap_sem);
}
up_write(&mm1->mmap_sem);

generate a lockdep warning about circular locking dependence.

Call rwsem_acquire() with trylock set to 1.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/rwsem.c

@@ -60,7 +60,7 @@ int down_write_trylock(struct rw_semaphore *sem)
 	int ret = __down_write_trylock(sem);
 
 	if (ret == 1)
-		rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
+		rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
 	return ret;
 }