linux-kernel - Re: [PATCH v2 2/3] Documentation: locking: Add local_lock_nested

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Message-ID: <db8defe4-14bf-4060-803f-e8b09a941d42@redhat.com>
Date: Mon, 18 Aug 2025 14:06:39 -0400
From: Waiman Long <llong@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Cc: Boqun Feng <boqun.feng@...il.com>, Clark Williams <clrkwllms@...nel.org>,
 Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 John Ogness <john.ogness@...utronix.de>, Jonathan Corbet <corbet@....net>,
 Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Valentin Schneider <vschneid@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 2/3] Documentation: locking: Add local_lock_nested_bh()
 to locktypes

On 8/15/25 5:38 AM, Sebastian Andrzej Siewior wrote:
> local_lock_nested_bh() is used within networking where applicable.
> Document why it is used and how it behaves.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>   Documentation/locking/locktypes.rst | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
>
> diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
> index 80c914f6eae7a..37b6a5670c2fa 100644
> --- a/Documentation/locking/locktypes.rst
> +++ b/Documentation/locking/locktypes.rst
> @@ -204,6 +204,27 @@ per-CPU data structures on a non PREEMPT_RT kernel.
>   local_lock is not suitable to protect against preemption or interrupts on a
>   PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.
>   
> +CPU local scope and bottom-half
> +-------------------------------
> +
> +Per-CPU variables that are accessed only in softirq context should not rely on
> +the assumption that this context is implicitly protected due to being
> +non-preemptible. In a PREEMPT_RT kernel, softirq context is preemptible, and
> +synchronizing every bottom-half-disabled section via implicit context results
> +in an implicit per-CPU "big kernel lock."
> +
> +A local_lock_t together with local_lock_nested_bh() and
> +local_unlock_nested_bh() for locking operations help to identify the locking
> +scope.
> +
> +When lockdep is enabled, these functions verify that data structure access
> +occurs within softirq context.
> +Unlike local_lock(), local_unlock_nested_bh() does not disable preemption and
> +does not add overhead when used without lockdep.

Should it be local_lock_nested_bh()? It doesn't make sense to compare 
local_unlock_nested_bh() against local_lock(). In a PREEMPT_RT kernel, 
local_lock() disables migration but not preemption.

Cheers,
Longman

> +
> +On a PREEMPT_RT kernel, local_lock_t behaves as a real lock and
> +local_unlock_nested_bh() serializes access to the data structure, which allows
> +removal of serialization via local_bh_disable().
>   
>   raw_spinlock_t and spinlock_t
>   =============================