linux-kernel - Re: [RFC] kernel/lockdep: use BFS(breadth-first search) algorithm to search target

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Date:	Tue, 26 May 2009 15:59:34 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] kernel/lockdep: use BFS(breadth-first search) algorithm
 to search target

On Tue, 2009-05-26 at 21:54 +0800, Ming Lei wrote:
> Hi,All
> 
> Currently lockdep uses recursion DFS(depth-first search) algorithm to
> search target in checking lock circle(check_noncircular()),irq-safe
> -> irq-unsafe(check_irq_usage()) and irq inversion when adding a new
> lock dependency. I plan to replace the current DFS with BFS, based on
> the following consideration:
> 
> 	1,no loss of efficiency, no matter DFS or BFS, the running time
> 	are O(V+E) (V is vertex count, and E is edge count of one
> 	graph);
> 
> 	2,BFS may be easily implemented by circular queue and consumes
> 	much less kernel stack space than DFS for DFS is implemented by
> 	recursion, we know kernel stack is very limited, eg. 4KB.
> 
> 	3, The shortest path can be obtained by BFS if the target is
> 	found, but can't be got by DFS. By the shortest path, we can
> 	shorten the lock dependency chain and help to troubleshoot lock
> 	problem easier than before.
> 
> Any suggestions, objections or viewpoint?

Ah, replace the full cycle detection might be worth it, esp with that
pre-allocated stack you used. Its all serialized on the graph lock
anyway.

I'm not sure about 3, though, since we search on adding a each new
dependency we'll only ever have a choice between cycles when one new
dependency generates two cycles at the same time. Something I think is
rare.

But yes, it wuold be nice to get rid of the current recursive algorithm
there.

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