Date: Mon, 29 Oct 2007 08:06:04 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, "George G. Davis" <gdavis@mvista.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       linux-fsdevel@vger.kernel.org
Subject: Re: [RFC, PATCH] locks: remove posix deadlock detection
Message-ID: <20071029080604.508b19ce@the-village.bc.nu>
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On Sun, 28 Oct 2007 13:43:21 -0400
"J. Bruce Fields" <bfields@fieldses.org> wrote:

> From: J. Bruce Fields <bfields@citi.umich.edu>
> 
> 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 seems unlikely to be useful to applications:
> 	- The difficulty of implementation has kept standards from
> 	  requiring it.  (E.g. SUSv3 : "Since implementation of full
> 	  deadlock detection is not always feasible, the [EDEADLK] error
> 	  was made optional.")  So portable applications may not be able to
> 	  depend on it.
> 	- It only detects deadlocks that involve nothing but local posix
> 	  file locks; deadlocks involving network filesystems or other kinds
> 	  of locks or resources are missed.
> 
> It therefore seems best to remove deadlock detection.
> 
> Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>


NAK. This is an ABI change and one that was rejected before when this was
last discussed in detail. Moving it out of BKL makes a ton of sense, even
adding a "don't check" flag makes a lot of sense. Removing the checking
does not.

I'd much rather see


	if (flags & FL_NODLCHECK)
		posix_deadlock_detect(....)


The failure case for removing this feature is obscure and hard to debug
application hangs for the afflicted programs - not nice for users at all.

Alan
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