Re: [RFC] Lockdep VS rw-semaphores strangeness

[Pavel Emelianov <xemul@sw.ru>]

Arjan van de Ven wrote:
> Pavel Emelianov wrote:
>> 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.
>>
> btw why is that a fake warning? what is your guarantee that you never
> ever do it in the opposite direction from the other thread as well?
> 

down_write(&mm->mmap_sem);
if (down_write_trylock(&another_mm->mmap_sem)) {
      ...
      up_write(...);
}
up_write(&mm->mmap_sem);

will never deadlock as if I have another thread is doing

down_write(&another_mm->mmap_sem);
if (down_write_trylock(&mm->mmap_sem)) {
      ...
      up_write(...);
}
up_write(&another_mm->mmap_sem);

booth will fail to trylock and go on.