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

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Message-ID: <8f36131b-e326-4d0e-96d6-9067b6c439d8@redhat.com>
Date: Tue, 19 Aug 2025 13:38:55 -0400
From: Waiman Long <llong@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Waiman Long <llong@...hat.com>
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, 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 8/19/25 6:00 AM, Sebastian Andrzej Siewior wrote:
> On 2025-08-18 14:06:39 [-0400], Waiman Long wrote:
>>> 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.
> Yes, it should have been the lock and not the unlock part. I mention
> just preemption part here because it focuses on the !RT part compared to
> local_lock() and that it adds no overhead.
> The PREEMPT_RT part below mentions that it behaves as a real lock so
> that should be enough (not to mention the migration part (technically
> migration must be already disabled so we could omit disabling migration
> here but it is just a counter increment/ decrement at this point so we
> don't win much by doing so)).
>
> I made the following:
>
> @@ -219,11 +219,11 @@ 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
> +Unlike local_lock(), local_lock_nested_bh() does not disable preemption and
>   does not add overhead when used without lockdep.
>   
>   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
> +local_lock_nested_bh() serializes access to the data structure, which allows
>   removal of serialization via local_bh_disable().
>   
>   raw_spinlock_t and spinlock_t
>
> Good?

LGTM, thanks!

Cheers,
Longman