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From: Thomas Gleixner <tglx@linutronix.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
 linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Cc: "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>, Peter
 Zijlstra <peterz@infradead.org>, Waiman Long <longman@redhat.com>, Will
 Deacon <will@kernel.org>, Sebastian Andrzej Siewior
 <bigeasy@linutronix.de>
Subject: Re: [PATCH v5 net-next 02/15] locking/local_lock: Add local nested
 BH locking infrastructure.
In-Reply-To: <20240607070427.1379327-3-bigeasy@linutronix.de>
References: <20240607070427.1379327-1-bigeasy@linutronix.de>
 <20240607070427.1379327-3-bigeasy@linutronix.de>
Date: Fri, 07 Jun 2024 15:55:22 +0200
Message-ID: <875xuk26yd.ffs@tglx>
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On Fri, Jun 07 2024 at 08:53, 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.
> 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>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>