linux-kernel - Re: [PATCH v2] rcu: Remove redundant rcu_read_lock/unlock() in spin

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Message-ID: <CAErzpmsW7=3RmLZxByxVD+vD=FV0YDF6POHVZZce784r7jMQyg@mail.gmail.com>
Date: Tue, 16 Sep 2025 09:55:25 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: tj@...nel.org, tony.luck@...el.com, jani.nikula@...ux.intel.com, 
	ap420073@...il.com, jv@...sburgh.net, freude@...ux.ibm.com, bcrl@...ck.org, 
	trondmy@...nel.org, longman@...hat.com, kees@...nel.org, 
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, 
	linux-nfs@...r.kernel.org, linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, netdev@...r.kernel.org, 
	intel-gfx@...ts.freedesktop.org, linux-acpi@...r.kernel.org, 
	linux-s390@...r.kernel.org, cgroups@...r.kernel.org, 
	Hillf Danton <hdanton@...a.com>, "Paul E . McKenney" <paulmck@...nel.org>, 
	pengdonglin <pengdonglin@...omi.com>
Subject: Re: [PATCH v2] rcu: Remove redundant rcu_read_lock/unlock() in
 spin_lock critical sections

On Mon, Sep 15, 2025 at 10:40 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2025-09-15 21:47:29 [+0800], pengdonglin wrote:
> > From: pengdonglin <pengdonglin@...omi.com>
> >
> > Per Documentation/RCU/rcu_dereference.rst [1], since Linux 4.20's RCU
> > consolidation [2][3], RCU read-side critical sections include:
> >   - Explicit rcu_read_lock()
> >   - BH/interrupt/preemption-disabling regions
> >   - Spinlock critical sections (including CONFIG_PREEMPT_RT kernels [4])
> >
> > Thus, explicit rcu_read_lock()/unlock() calls within spin_lock*() regions are redundant.
> > This patch removes them, simplifying locking semantics while preserving RCU protection.
> >
> > [1] https://elixir.bootlin.com/linux/v6.17-rc5/source/Documentation/RCU/rcu_dereference.rst#L407
> > [2] https://lore.kernel.org/lkml/20180829222021.GA29944@linux.vnet.ibm.com/
> > [3] https://lwn.net/Articles/777036/
> > [4] https://lore.kernel.org/lkml/6435833a-bdcb-4114-b29d-28b7f436d47d@paulmck-laptop/
>
> What about something like this:
>
>   Since commit a8bb74acd8efe ("rcu: Consolidate RCU-sched update-side
>   function definitions") there is no difference between rcu_read_lock(),
>   rcu_read_lock_bh() and rcu_read_lock_sched() in terms of RCU read
>   section and the relevant grace period. That means that spin_lock(),
>   which implies rcu_read_lock_sched(), also implies rcu_read_lock().
>
>   There is no need no explicitly start a RCU read section if one has
>   already been started implicitly by spin_lock().
>
>   Simplify the code and remove the inner rcu_read_lock() invocation.
>
>
> The description above should make it clear what:
> - the intention is
> - the proposed solution to it and why it is correct.

Thanks, that's much clearer. I'll use this commit message in v3.

>
> You can't send a patch like this. You need to split it at the very least
> by subsystem. The networking bits need to follow to follow for instance
>    Documentation/process/maintainer-netdev.rst

Thanks, I will split this into a series for v3.

>
> and so on.
>
> Sebastian