[PATCH v3 0/7] Introduce local_lock()

[Sebastian Andrzej Siewior]

This is v3 of the local_lock() series. The v2 can be found at 

   https://lore.kernel.org/lkml/20200524215739.551568-1-bigeasy@xxxxxxxxxxxxx/

v2âv3:
  - Use `local_lock_t' instead of `struct local_lock' because it is
    tiny data structure in general (similar to spinlock_t). Use also
    consistent file names `local_lock.h'.

  - Export the data structure in radix-tree so that the `lock' member
    can be accessed externally. The common case of 'local_unlock()' (no
    lockdep, no preemption) will then be optimized away. Otherwise
    `idr_preload_end()' will be a function containing only a return
    opcode.

  - Reorganize the struct member names in mm/swap and connector/cn_proc.

  - Make the `lock' member comes before the member that it aims to
    protect.

  - Two hunks from patch #6 appeared under mysteries circumstances in
    patch #7. They have been moved back to patch #6.
    Also applied comments to patch #7 as suggested by Ingo.

v1âv2:
  - Remove static initializer so a local_lock is not used a single
    per-CPU variable but a 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