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    SubjectRe: [PATCH v6 1/2] pid: Replace pid bitmap implementation with IDR API
    On 10/12, Andrew Morton wrote:
    >
    > On Wed, 11 Oct 2017 20:54:32 -0400 Rik van Riel <riel@surriel.com> wrote:
    >
    > > Documenting the locking on all the existing code, plus the
    > > new code, seems a little out of scope of an Outreachy
    > > internship...
    >
    > I'm not referring to all the existing code! Just this new
    > pid_namespace.idr's locking. If it was protected by
    > spin_lock_irq(&pidmap_lock) everywhere then an explanation wouldn't be
    > needed. But we have this oddball site where pidmap_lock isn't taken
    > but it uses rcu_read_lock() which is surprising to say the least.
    > Readers could be forgiven for thinking that is a bug.

    Actually this all looks simple, or I missed something...

    pid_namespace.idr is protected by pidmap_lock, it is always modified
    with this lock held.

    idr_find/idr_find_ext/etc can be called under rcu_read_lock() simply
    because idr/radix_tree is rcu-safe and this is already documented in
    idr.h, say, the comment above idr_find() says

    * This function can be called under rcu_read_lock(), given that the leaf
    * pointers lifetimes are correctly managed.

    Oleg.

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