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Date: Thu, 6 Jun 2024 09:52:44 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>,
	Daniel Bristot de Oliveira <bristot@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Eric Dumazet <edumazet@google.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v4 net-next 01/14] locking/local_lock: Add local nested
 BH locking infrastructure.
Message-ID: <20240606075244.GB8774@noisy.programming.kicks-ass.net>
References: <20240604154425.878636-1-bigeasy@linutronix.de>
 <20240604154425.878636-2-bigeasy@linutronix.de>
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In-Reply-To: <20240604154425.878636-2-bigeasy@linutronix.de>

On Tue, Jun 04, 2024 at 05:24:08PM +0200, Sebastian Andrzej Siewior wrote:
> Add local_lock_nested_bh() locking. It is based on local_lock_t and the
> naming follows the preempt_disable_nested() example.
> 
> For !PREEMPT_RT + !LOCKDEP it is a per-CPU annotation for locking
> assumptions based on local_bh_disable(). The macro is optimized away
> during compilation.
> For !PREEMPT_RT + LOCKDEP the local_lock_nested_bh() is reduced to
> the usual lock-acquire plus lockdep_assert_in_softirq() - ensuring that
> BH is disabled.
> 
> For PREEMPT_RT local_lock_nested_bh() acquires the specified per-CPU
> lock. It does not disable CPU migration because it relies on
> local_bh_disable() disabling CPU migration.

should we assert this? lockdep_assert(current->migration_disabled) or
somesuch should do, rite?

> With LOCKDEP it performans the usual lockdep checks as with !PREEMPT_RT.
> Due to include hell the softirq check has been moved spinlock.c.
> 
> The intention is to use this locking in places where locking of a per-CPU
> variable relies on BH being disabled. Instead of treating disabled
> bottom halves as a big per-CPU lock, PREEMPT_RT can use this to reduce
> the locking scope to what actually needs protecting.
> A side effect is that it also documents the protection scope of the
> per-CPU variables.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Otherwise I suppose sp.. not a fan of the whole nested thing, but I
don't really have an alternative proposal so yeah, whatever :-)