Re: workqueue thing

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 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.

Do you mean like the annotations we added in:

  4e6045f: workqueue: debug flushing deadlocks with lockdep
  a67da70: workqueues: lockdep annotations for flush_work()

?

It looks like this currently in the worklet:

                lock_map_acquire(&cwq->wq->lockdep_map);
                lock_map_acquire(&lockdep_map);
                f(work);
                lock_map_release(&lockdep_map);
                lock_map_release(&cwq->wq->lockdep_map);

and like this in flush:

        lock_map_acquire(&wq->lockdep_map);
        lock_map_release(&wq->lockdep_map);
        for_each_cpu(cpu, cpu_map)
                flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));

We basically track the implicit dependencies even if they are not executed 
(only theoretically possible) - and we subsequently caught a few bugs that 
way.

Or did you have some other dependency in mind?

	Ingo
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