linux-kernel - Re: [PATCH] rcu-tasks: Remove unnecessary smp_store operations in the cblist_init

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Message-ID: <9254e0cfc06f9820d77113a4be5d3f6d6edd28cd@linux.dev>
Date: Wed, 31 Dec 2025 07:55:00 +0000
From: "Zqiang" <qiang.zhang@...ux.dev>
To: paulmck@...nel.org
Cc: frederic@...nel.org, neeraj.upadhyay@...nel.org, joelagnelf@...dia.com,
 rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu-tasks: Remove unnecessary smp_store operations in
 the cblist_init_generic()

> 
> On Sun, Dec 14, 2025 at 10:49:04AM +0800, Zqiang wrote:
> 
> > 
> > The cblist_init_generic() is executed during the CPU early boot
> >  phase due to commit:30ef09635b9e ("rcu-tasks: Initialize callback
> >  lists at rcu_init() time"), at this time, only one boot CPU is
> >  online. this commit therefore use WRITE_ONCE() to replace the
> >  smp_store_release() to assigning rtp->percpu_enqueue_lim in the
> >  cblist_init_generic().
> >  
> >  Signed-off-by: Zqiang <qiang.zhang@...ux.dev>
> > 
> Good eyes, thank you!
> 
> But in that case, we are running on a single CPU with interrupts disables,
> so do we even need WRITE_ONCE()?

I missed the interrupts disabled, It seems like there are no need use WRITE_ONCE()
and include above, I will update and resend.

Thanks
Zqiang




> 
>  Thanx, Paul
> 
> > 
> > ---
> >  kernel/rcu/tasks.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >  
> >  diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> >  index 76f952196a29..f763f60d5e50 100644
> >  --- a/kernel/rcu/tasks.h
> >  +++ b/kernel/rcu/tasks.h
> >  @@ -292,7 +292,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> >  shift++;
> >  WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
> >  WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
> >  - smp_store_release(&rtp->percpu_enqueue_lim, lim);
> >  + WRITE_ONCE(rtp->percpu_enqueue_lim, lim);
> >  
> >  pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d rcu_task_cpu_ids=%d.\n",
> >  rtp->name, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim),
> >  -- 
> >  2.48.1
> >
>