linux-kernel - [PATCH v2 0/5] seqlock: Introduce PREEMPT

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Date:   Fri,  4 Sep 2020 17:32:26 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>
Subject: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

Hi,

Changelog-v2
============

  - Standardize on seqcount_LOCKNAME_t as the canonical reference for
    sequence counters with associated locks, instead of v1
    seqcount_LOCKTYPE_t.

  - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
    property accessors.

  - Touch-up the lock-unlock rationale for more clarity. Enforce writer
    non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".

Cover letter (v1)
=================

https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de

Preemption must be disabled before entering a sequence counter write
side critical section.  Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick.  If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).

To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.

Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.

8<--------------

Ahmed S. Darwish (5):
  seqlock: seqcount_LOCKNAME_t: Standardize naming convention
  seqlock: Use unique prefix for seqcount_t property accessors
  seqlock: seqcount_t: Implement all read APIs as statement expressions
  seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
  seqlock: PREEMPT_RT: Do not starve seqlock_t writers

 include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
 1 file changed, 167 insertions(+), 114 deletions(-)

base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
--
2.28.0