linux-kernel - [RFC] Lockdep VS rw-semaphores strangeness

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Date:	Fri, 06 Apr 2007 14:14:28 +0400
From:	Pavel Emelianov <xemul@...ru>
To:	Ingo Molnar <mingo@...hat.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC] Lockdep VS rw-semaphores strangeness

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?

View attachment "diff-lockdep-rwsem-trylock" of type "text/plain" (357 bytes)