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

[Ming Lei]

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?

Thanks,
-- 
Lei Ming
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