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

Alan Cox
2007-10-29T08:04:41Z

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