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

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Date:	Fri, 06 Apr 2007 18:19:09 +0400
From:	Pavel Emelianov <xemul@...ru>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Lockdep VS rw-semaphores strangeness

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.
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