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Date: Tue, 10 Apr 2007 11:37:00 +0400
From: Pavel Emelianov <xemul@sw.ru>
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To: Arjan van de Ven <arjan@linux.intel.com>
CC: Andrew Morton <akpm@osdl.org>, Ingo Molnar <mingo@redhat.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Lockdep treats down_write_trylock like regular down_write
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Arjan van de Ven wrote:
> Pavel Emelianov wrote:
>> 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.
> 
> please show me why this is safe, especially if you intermix it with
> down_read()'s... like copy_to_user and co may do.

This is safe as once the task locks the mm1->mmap_sem and gets
into '...' place it is *running* and will release booth semaphores
for sure.

> this feels like a very unsafe construct to me...
>