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    SubjectRe: workqueue thing
    On Fri, 2009-12-18 at 07:30 -0800, Linus Torvalds wrote:
    > 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).

    That's exactly what it does..


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