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    SubjectRe: [PATCH] spinlock: __raw_spin_is_locked() should return true for UP
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    On Wed, 2009-08-19 at 11:31 +0200, Olivier Galibert wrote:
    > On Tue, Aug 18, 2009 at 07:40:16PM -0700, Linus Torvalds wrote:
    > >
    > >
    > > On Tue, 18 Aug 2009, Kumar Gala wrote:
    > > >
    > > > I agree its a little too easy to abuse spin_is_locked. However we should be
    > > > consistent between spin_is_locked on UP between with and without
    > > > CONFIG_DEBUG_SPINLOCK enabled.
    > >
    > > No we shouldn't.
    > >
    > > With CONFIG_DEBUG_SPINLOCK, you have an actual lock variable for debugging
    > > purposes, so spin_is_locked() can clearly return a _valid_ answer, and
    > > should do so.
    > >
    > > Without DEBUG_SPINLOCK, there isn't any answer to return.
    > >
    > > So there's no way we can or should be consistent. In one case an answer
    > > exists, in another one the answer is meaningless and doesn't exist.
    >
    > I always thought behaviour should be consistent between code with
    > debugging on and code without. Otherwise you may end up with cases of
    > "it starts working when I turn on debugging" which are a pain to fix.
    > Has something changed?
    >
    > Or in other words, do you think lockdep should try solving deadlocks
    > instead of just reporting them for instance?

    The point is spin_is_locked() is a broken interface in that respect. Its
    plain impossible to give the right answer.


    Suppose there's code doing:

    /*
    * Ensure we don't have foo lock taken, because that would cause
    * lock inversion under bar lock.
    */
    BUG_ON(spin_is_locked(&foo));
    spin_lock(&bar);

    and other code doing:

    /*
    * Ensure we've got foo locked because it protects bar
    */
    BUG_ON(!spin_is_locked(&foo));
    bar = fancy;

    What value should you return when locks don't exist (which is the case
    for UP)?


    There simply is no right answer other than: don't use spin_is_locked().


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