linux-kernel - [PATCH 0/7 v2] Introduce local

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Date:   Sun, 24 May 2020 23:57:32 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: [PATCH 0/7 v2] Introduce local_lock()

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