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

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Date: Mon, 06 May 2024 10:43:49 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	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>, Peter
 Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 net-next 00/15] locking: Introduce nested-BH locking.

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