On Fri, 2024-05-03 at 20:25 +0200, Sebastian Andrzej Siewior wrote:

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.

Let me rephrase to check I understood the plan correctly.

The idea is to pair 'bare' local_bh_{disable,enable} with local lock
and late make local_bh_{disable,enable} no ops (on RT).

With 'bare' I mean not followed by a spin_lock() - which is enough to
ensure mutual exclusion vs BH on RT build - am I correct?

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.

AFAICS there are a bunch of local_bh_* call-sites under 'net' matching
the above description and not addressed here. Is this series supposed
to cover 'net' fully?

Could you please include the diffstat for the whole series? I
think/hope it will help catching the full picture more easily.

Note that some callers use local_bh_disable(), no additional lock, and
there is no specific struct to protect, but enforce explicit
serialization vs bh to a bunch of operation, e.g.  the
local_bh_disable() in inet_twsk_purge().

I guess such call site should be handled, too?

Thanks!

Paolo