linux-kernel - Re: [patch 2/3] splice: implement default splice

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Date:	Wed, 13 May 2009 11:01:43 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jens.axboe@...cle.com
CC:	akpm@...ux-foundation.org, miklos@...redi.hu, max@...mpel.org,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/3] splice: implement default splice_read method

On Wed, 13 May 2009, Jens Axboe wrote:
> On Tue, May 12 2009, Andrew Morton wrote:
> > On Thu, 07 May 2009 15:37:36 +0200 Miklos Szeredi <miklos@...redi.hu> wrote:
> > 
> > > +	for (i = 0; i < spd.nr_pages; i++) {
> > > +		kunmap(pages[i]);
> > 
> > It is deadlockable if any thread of control holds more than a single
> > kmap at a time.
> > 
> > Because there are a finite number of kmaps available, and if one is
> > unavailable, kmap() waits for one to become free.  If the number of
> > waiting threads equals the number of available slots, nobody makes any
> > progress.

Ouch.

> 
> Good catch, that will not work reliably. I've applied the below.

Thanks.

The bigger problem is that the default_file_splice_write()
implementation in the other patch does the same (it calls
buf->ops->map() on all buffers).

Hmm.  Simple solution would be to do a write() for each buffer.  But
this only affects HIGHMEM kernels, so it's a bit pointless to do that
on all archs.  Sigh...

Miklos
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