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

[Sebastian Andrzej Siewior]

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@xxxxxxxxxxxxx/:
- 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@xxxxxxxxxxxxx/

  and then updated

        v2: https://lore.kernel.org/all/20240229183109.646865-1-bigeasy@xxxxxxxxxxxxx/
	  - 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