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    Subject[PATCH 0/7 v2] Introduce local_lock()
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    This is v2 of the local_lock() series. The v1 can be found at 

    https://lore.kernel.org/lkml/20200519201912.1564477-1-bigeasy@linutronix.de/

    v1…v2:
    - Remove static initializer so a local_lock is not used as a single
    per-CPU variable but as a member of an existing structure, that is
    used per-CPU.

    - Use LD_WAIT_CONFIG as wait-type in the dep_map.

    - Expect a pointer like value as argument (same as this_cpu_ptr()).

    - Drop the SRCU patch. A different sollution is worked on.

    - Drop the zswap patch. That code part will be reworked.


    preempt_disable() and local_irq_disable/save() are in principle per CPU big
    kernel locks. This has several downsides:

    - The protection scope is unknown

    - Violation of protection rules is hard to detect by instrumentation

    - For PREEMPT_RT such sections, unless in low level critical code, can
    violate the preemptability constraints.

    To address this PREEMPT_RT introduced the concept of local_locks which are
    strictly per CPU.

    The lock operations map to preempt_disable(), local_irq_disable/save() and
    the enabling counterparts on non RT enabled kernels.

    If lockdep is enabled local locks gain a lock map which tracks the usage
    context. This will catch cases where an area is protected by
    preempt_disable() but the access also happens from interrupt context. local
    locks have identified quite a few such issues over the years, the most
    recent example is:

    b7d5dc21072cd ("random: add a spinlock_t to struct batched_entropy")

    Aside of the lockdep coverage this also improves code readability as it
    precisely annotates the protection scope.

    PREEMPT_RT substitutes these local locks with 'sleeping' spinlocks to
    protect such sections while maintaining preemtability and CPU locality.

    The followin series introduces the infrastructure including
    documentation and provides a couple of examples how they are used to
    adjust code to be RT ready.

    Sebastian

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