linux-kernel - Re: [RFC, PATCH] locks: remove posix deadlock detection

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Date:	Sun, 28 Oct 2007 18:40:52 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Matthew Wilcox <matthew@....cx>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	"George G. Davis" <gdavis@...sta.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC, PATCH] locks: remove posix deadlock detection

On Sun, 28 Oct 2007 12:27:32 -0600
Matthew Wilcox <matthew@....cx> wrote:

> On Sun, Oct 28, 2007 at 01:43:21PM -0400, J. Bruce Fields wrote:
> > We currently attempt to return -EDEALK to blocking fcntl() file locking
> > requests that would create a cycle in the graph of tasks waiting on
> > locks.
> > 
> > This is inefficient: in the general case it requires us determining
> > whether we're adding a cycle to an arbitrary directed acyclic graph.
> > And this calculation has to be performed while holding a lock (currently
> > the BKL) that prevents that graph from changing.
> > 
> > It has historically been a source of bugs; most recently it was noticed
> > that it could loop indefinitely while holding the BKL.
> 
> It can also return -EDEADLK spuriously.  So yeah, just kill it.

NAK. This is an ABI change. It was also comprehensively rejected before
because

- EDEADLK behaviour is ABI
- EDEADLK behaviour is required by SuSv3
- We have no idea what applications may rely on this behaviour.

and also SuSv3 is required by LSB

See the thread
	http://osdir.com/ml/file-systems/2004-06/msg00017.html

so we need to fix the bugs - the lock usage and the looping. At that
point it merely becomes a performance concern to those who use it, which
is the proper behaviour. If you want a faster non-checking one use
flock(), or add another flag that is a Linux "don't check for deadlock"

Alan
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