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

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Message-ID: <20250912213531.7-YeRBeD@linutronix.de>
Date: Fri, 12 Sep 2025 23:35:31 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Waiman Long <llong@...hat.com>
Cc: pengdonglin <dolinux.peng@...il.com>, 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, 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,
	pengdonglin <pengdonglin@...omi.com>,
	"Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] rcu: Remove redundant rcu_read_lock/unlock() in
 spin_lock critical sections

On 2025-09-12 17:13:09 [-0400], Waiman Long wrote:
> On 9/12/25 2:50 AM, pengdonglin wrote:
> > From: pengdonglin <pengdonglin@...omi.com>
> > 
> > When CONFIG_PREEMPT_RT is disabled, spin_lock*() operations implicitly
> > disable preemption, which provides RCU read-side protection. When
> > CONFIG_PREEMPT_RT is enabled, spin_lock*() implementations internally
> > manage RCU read-side critical sections.
> 
> I have some doubt about your claim that disabling preemption provides RCU
> read-side protection. It is true for some flavors but probably not all. I do
> know that disabling interrupt will provide RCU read-side protection. So for
> spin_lock_irq*() calls, that is valid. I am not sure about spin_lock_bh(),
> maybe it applies there too. we need some RCU people to confirm.

The claim is valid since Paul merged the three flavours we had. Before
that preempt_disable() (and disabling irqs) would match
rcu_read_lock_sched(). rcu_read_lock() and rcu_read_lock_bh() were
different in terms of grace period and clean up.
So _now_ we could remove it if it makes things easier.

> Cheers,
> Longman

Sebastian