[RFC] Lockdep VS rw-semaphores strangeness

[Pavel Emelianov <xemul@sw.ru>]

Hello Ingo, Arjan.

I'm playing with lockdep and have a question about rw-sems.

down_read_trylock() looks like
int down_read_trylock(struct rw_semaphore *sem)
{
        int ret = __down_read_trylock(sem);

        if (ret == 1)
                rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
        return ret;
}
i.e. it calls rwsem_acquire_read() with trylock == 1.

But down_write_trylock() -
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_);
        return ret;
}
- calls lockdep with trylock set to 0. Why?

I've already caught a fake warning when trying to write-lock
an mm->mmap_sem with another mm's mmap_sem write-locked. With the
patch attached everything works, fine.

Did I miss something?

--- ./kernel/rwsem.c.pbonrem	2007-03-06 19:09:50.000000000 +0300
+++ ./kernel/rwsem.c	2007-04-06 14:02:18.000000000 +0400
@@ -60,7 +60,7 @@ int down_write_trylock(struct rw_semapho
 	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;
 }