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

From: Peter Zijlstra
Date: Tue May 26 2009 - 09:59:45 EST

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

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

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

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