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    SubjectRe: [RFC] kernel/lockdep: use BFS(breadth-first search) algorithm to search target
    From
    2009/5/26 Peter Zijlstra <a.p.zijlstra@chello.nl>:
    > 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.

    Another case, there are several lock_list instances in one lock dependency graph
    ,which all points to one lock_class, BFS can find the one with
    shortest distance,but
    DFS can't. The scenario should be common, right?

    Thanks.

    >>
    >> 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.
    >
    >



    --
    Lei Ming
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