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    SubjectRe: hrtimers: simplify lockdep handling

    Hi Oleg,

    I'm currently porting -rt to 26-rc7 and I came across this change:

    Commit: 8e60e05fdc7344415fa69a3883b11f65db967b47

    With the

    - double_spin_lock(&new_base->lock, &old_base->lock,
    - smp_processor_id() < cpu);
    + spin_lock(&new_base->lock);
    + spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);

    What's the reason that this is possible? Is it because the migration
    happens only on CPU hotplugging and that the CPU hotplugging code has
    locks that would prevent a reversal of the lock taking?

    I'm not arguing that the code is incorrect, but this looks like a subtlety
    that can bite us later.

    In other words, we really need comments around this code to explain to
    casual viewers why this code is not deadlock prone. The change log here
    and for 0d180406f2914aea3a78ddb880e2fe9ac78a9372 does not explain why the
    straight forward taking of the locks is OK.


    -- Steve

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