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

From: Ming Lei
Date: Tue May 26 2009 - 20:31:02 EST

2009/5/26 Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>:
> 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?


>> 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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