Messages in this thread | | | Date | Fri, 18 Dec 2009 07:30:55 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: workqueue thing |
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On Fri, 18 Dec 2009, Peter Zijlstra wrote:
> > r1. The first design goal of cmwq is solving the issues the current > > workqueue implementation has including hard to detect > > deadlocks, > > lockdep is quite proficient at finding these these days.
I don't think so.
The reason it is not is that workqueues fundamentally do _different_ things in the same context, adn lockdep has no clue what-so-ever.
IOW, if you hold a lock, and then do 'flush_workqueue()', lockdep has no idea that maybe one of the entries on a workqueue might need the lock that you are holding. But I don't think lockdep sees the dependency that gets created by the flush - because it's not a direct code execution dependency.
It's not a deadlock _directly_ due to lock ordering, but indirectly due to waiting for unrelated code that needs locks.
Now, maybe lockdep could be _taught_ to consider workqueues themselves to be 'locks', and ordering those pseudo-locks wrt the real locks they take. So if workqueue Q takes lock A, the fact that it is _taken_ in a workqueue makes the ordering be Q->A. Then, if somebody does a "flush_workqueue" while holding lock B, the flush implies a "lock ordering" of B->Q (where "Q" is the set of all workqueues that could be flushed).
Linus
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