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    SubjectRe: workqueue thing


    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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