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:
>> 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
>> 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
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
> 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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/