linux-kernel - [PATCH v2 net-next 00/15] locking: Introduce nested-BH locking.

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Date: Fri,  3 May 2024 20:25:04 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>,
	Will Deacon <will@...nel.org>
Subject: [PATCH v2 net-next 00/15] locking: Introduce nested-BH locking.

Disabling bottoms halves acts as per-CPU BKL. On PREEMPT_RT code within
local_bh_disable() section remains preemtible. As a result high prior
tasks (or threaded interrupts) will be blocked by lower-prio task (or
threaded interrupts) which are long running which includes softirq
sections.

The proposed way out is to introduce explicit per-CPU locks for
resources which are protected by local_bh_disable() and use those only
on PREEMPT_RT so there is no additional overhead for !PREEMPT_RT builds.

The series introduces the infrastructure and converts large parts of
networking which is largest stake holder here. Once this done the
per-CPU lock from local_bh_disable() on PREEMPT_RT can be lifted.

v1…v2 https://lore.kernel.org/all/20231215171020.687342-1-bigeasy@linutronix.de/:
- Jakub complained about touching networking drivers to make the
  additional locking work. Alexei complained about the additional
  locking within the XDP/eBFP case.
  This led to a change in how the per-CPU variables are accessed for the
  XDP/eBPF case. On PREEMPT_RT the variables are now stored on stack and
  the task pointer to the structure is saved in the task_struct while
  keeping every for !RT unchanged. This was proposed as a RFC in
  	v1: https://lore.kernel.org/all/20240213145923.2552753-1-bigeasy@linutronix.de/

  and then updated

        v2: https://lore.kernel.org/all/20240229183109.646865-1-bigeasy@linutronix.de/
	  - Renamed the container struct from xdp_storage to bpf_net_context.
            Suggested by Toke Høiland-Jørgensen.
	  - Use the container struct also on !PREEMPT_RT builds. Store the
	    pointer to the on-stack struct in a per-CPU variable. Suggested by
            Toke Høiland-Jørgensen.

  This reduces the initial queue from 24 to 15 patches.

- There were complains about the scoped_guard() which shifts the whole
  block and makes it harder to review because the whole gets removed and
  added again. The usage has been replaced with local_lock_nested_bh()+
  its unlock counterpart.

Sebastian