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    SubjectRe: 2.6.24-rc6: possible recursive locking detected
    On 01/13, Peter Zijlstra wrote:
    >
    > On Mon, 2008-01-07 at 20:49 +0300, Oleg Nesterov wrote:
    >
    > > On 01/07, Oleg Nesterov wrote:
    > > >
    > > > Consider this "just for illustration" patch,
    > > >
    > > > --- t/kernel/lockdep.c 2007-11-09 12:57:31.000000000 +0300
    > > > +++ t/kernel/lockdep.c 2008-01-07 19:43:50.000000000 +0300
    > > > @@ -1266,10 +1266,13 @@ check_deadlock(struct task_struct *curr,
    > > > struct held_lock *prev;
    > > > int i;
    > > >
    > > > - for (i = 0; i < curr->lockdep_depth; i++) {
    > > > + for (i = curr->lockdep_depth; --i >= 0; ) {
    > > > prev = curr->held_locks + i;
    > > > if (prev->class != next->class)
    > > > continue;
    > > > +
    > > > + if (prev->trylock == -1)
    > > > + return 2;
    > > > /*
    > > > * Allow read-after-read recursion of the same
    > > > * lock class (i.e. read_lock(lock)+read_lock(lock)):
    > > > -------------------------------------------------------------------------
    > > >
    > > > Now,
    > > >
    > > > // trylock == -1
    > > > #define spin_mark_nested(l) \
    > > > lock_acquire(&(l)->dep_map, 0, -1, 0, 2, _THIS_IP_)
    > > > #define spin_unmark_nested(l) \
    > > > lock_release(&(l)->dep_map, 1, _THIS_IP_)
    > > >
    > > > and ep_poll_safewake() can do:
    > > >
    > > > /* Do really wake up now */
    > > > spin_mark_nested(&wq->lock);
    > > > wake_up(wq);
    > > > spin_unmark_nested(&wq->lock);
    > >
    > > seems to work. What do you think?
    >
    > I've been pondering this for a while, and some days I really like it,
    > some days I don't.
    >
    > The problem I have with it is that it becomes very easy to falsely
    > annotate problems away - its a very powerful annotation.

    Also, I don't like the overloading of ->trylock, this is really hackish.

    > I think I'll do wake_up_nested() for now and keep this around.

    Agreed.

    Perhaps it is a bit easier to use spin_lock_nested() + __wake_up_common()
    directly (we have a lot of wake_up_xxx helpers), but this is up to you.


    Offtopic question. Why do we have so many lockdep stuff in timer.c and hrtimer.c ?
    We never lock 2 bases at the same time, except in migrate_timers(). We can kill
    double_spin_lock() and base_lock_keys[] and just use spin_lock_nested in
    migrate_timers(), no?

    Oleg.



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