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

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Message-ID: <20080408213054.GA8791@2ka.mipt.ru>
Date:	Wed, 9 Apr 2008 01:30:54 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, Jens Axboe <axboe@...nel.dk>,
	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 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.

I have an even deeper hack to fix this issue: put a list of pipe
structures as a lru.next pointer, mapping lock can guard processing...
Sounds scary, want to try.

-- 
	Evgeniy Polyakov
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