linux-kernel - Re: [PATCH] lock: Add doc comments for spin_lock

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Message-ID: <957be7c9-7860-4824-b491-88cb9741dfab@redhat.com>
Date: Sat, 11 Oct 2025 22:31:17 -0400
From: Waiman Long <llong@...hat.com>
To: Daroc Alden <daroc@....net>, Waiman Long <llong@...hat.com>
Cc: corbet@....net, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 "open list:LOCKING PRIMITIVES" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lock: Add doc comments for spin_lock_irq()

On 10/11/25 2:28 PM, Daroc Alden wrote:
> On Fri, 10 Oct 2025 23:15:50 -0400
> Waiman Long <llong@...hat.com> wrote:
>
>> On 10/10/25 5:53 PM, Daroc Alden wrote:
>>> The commonly used spin_lock_irq(), spin_lock_irqsave(),
>>> spin_unlock_irq(), and spin_unlock_irqrestore() functions do not
>>> currently have any documentation; this commit adds kerneldoc
>>> comments to these four functions describing when their behavior and
>>> when they are appropriate to use.
>>>
>>> Signed-off-by: Daroc Alden <daroc@....net>
>> This patch looks fine. Just wonder why just
>> spin_lock_irq()/spin_lock_irqsave() and not
>> spin_lock()/spin_lock_bh() as these functions also don't have
>> kerneldoc comments. Also spin_lock_irqsave() is a macro and not
>> actually a function, maybe we should mention that in the comment.
>>
> Because I had to research spin_lock_irq()/spin_lock_irqsave() for a
> recent article, and therefore felt confident that I understood how they
> behaved and what should go in the doc comment.
>
> If you — as a more experienced kernel person — can describe how/why the
> _bh() variants are used, I'm happy to add doc comments for them as
> well. My current understanding is that they interact with whatever is
> left of the "big kernel lock". Is that right?

"bh" in spin_lock_bh() stands for bottom half which is essentially what 
what is being done in the softIRQ context. So spin_lock_bh() just 
prevents the softIRQ code from being executed. This is my understanding, 
but I may have missed other use cases of spin_lock_bh(). Others can 
chime in if there is more to say. Anyway, I am fine with adding more 
comments to spinlock code.

Cheers, Longman