On 9/12/25 5:35 PM, Sebastian Andrzej Siewior wrote:

On 2025-09-12 17:13:09 [-0400], Waiman Long wrote:

quoted

On 9/12/25 2:50 AM, pengdonglin wrote:

quoted

From: pengdonglin <redacted>

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.

Thanks for the clarification.

In this case, I think the patch description should mention the commit 
that unify the 3 RCU flavors to make sure that this patch won't be 
accidentally backport'ed to an older kernel without the necessary 
prerequisite commit(s).

Cheers,
Longman