From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch 2/3] splice: implement default splice_read method
Date: Tue, 12 May 2009 22:35:00 -0700
Message-ID: <20090512223500.d7ef4648.akpm@linux-foundation.org>
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Cc: jens.axboe@oracle.com, Max Kellermann <max@duempel.org>,
	torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
To: Miklos Szeredi <miklos@szeredi.hu>
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On Thu, 07 May 2009 15:37:36 +0200 Miklos Szeredi <miklos@szeredi.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.