netdev - Re: [take 2] Fix for the fundamental network/block layer race in sendfile().

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Date:	Wed, 9 Apr 2008 13:33:21 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [take 2] Fix for the fundamental network/block layer race in sendfile().

On Wed, Apr 09 2008, Evgeniy Polyakov wrote:
> On Tue, Apr 08, 2008 at 09:26:52PM +0400, Evgeniy Polyakov (johnpol@....mipt.ru) wrote:
> > >    Unless I mistaken, you store in page->lru.next some info to find 
> > > your pipe pointer, assuming it is unique for this page.
> > > 
> > > What happens if two threads are doing a splice()/sendfile() using the 
> > > same underlying (source) file (and same pages in this file)
> > 
> > Page will be referenced twice (for each thread) and each thread will
> > have own pipe_inode_info structure, so each one will sleep on own wait
> > queue and will be awakened separately, where its release counter will be
> > dropped.
> 
> I was wrong, here will be a problem - page can be shared between multiple
> threads, since it is unlocked after putting it into page cache, so it is
> not allowed to put into it some per-thread private data.

Threads or not, it does not matter. Even simpler, the same page could be
in multiple pipes. So that trick definitely will not work at all.

-- 
Jens Axboe

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