linux-kernel - Re: [PATCH v2] srcu/tiny: Remove preempt_disable/enable() in srcu_gp_start_if

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Message-ID: <ab97f38c-2750-4cd7-a9a4-6c25fc1cc45d@paulmck-laptop>
Date: Thu, 11 Sep 2025 05:06:11 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang.zhang@...ux.dev>
Cc: Joel Fernandes <joelagnelf@...dia.com>, frederic@...nel.org,
	neeraj.upadhyay@...nel.org, boqun.feng@...il.com, urezki@...il.com,
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] srcu/tiny: Remove preempt_disable/enable() in
 srcu_gp_start_if_needed()

On Thu, Sep 11, 2025 at 11:46:36AM +0000, Zqiang wrote:
> > 
> > On Thu, Sep 11, 2025 at 12:36:45AM +0000, Zqiang wrote:
> > 
> > > 
> > > On Wed, Sep 10, 2025 at 10:36:20AM -0400, Joel Fernandes wrote:
> > >  
> > >  > 
> > >  > [..]
> > >  > > kernel/rcu/srcutiny.c | 4 +---
> > >  > > 1 file changed, 1 insertion(+), 3 deletions(-)
> > >  > > 
> > >  > > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > >  > > index b52ec45698e8..b2da188133fc 100644
> > >  > > --- a/kernel/rcu/srcutiny.c
> > >  > > +++ b/kernel/rcu/srcutiny.c
> > >  > > @@ -181,10 +181,9 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > >  > > {
> > >  > > unsigned long cookie;
> > >  > > 
> > >  > > - preempt_disable(); // Needed for PREEMPT_LAZY
> > >  > > + lockdep_assert_preemption_disabled();
> > >  > 
> > >  > nit: Do we still want to keep the comment that the expectation of preemption
> > >  > being disabled is for the LAZY case?
> > >  > 
> > >  Good point, and I do believe that we do. Zqiang, any reason not to
> > >  add this comment back in?
> > >  
> > >  in rcu-tree, this commit:
> > >  
> > >  (935147775c977 "EXP srcu: Enable Tiny SRCU On all CONFIG_SMP=n kernels")
> > >  
> > >  make preempt disable needed for CONFIG_PREEMPT=y or CONFIG_PREEMPT_LAZY=y
> > >  when the CONFIG_SMP=n. do we need to replace "Needed for PREEMPT_LAZY"
> > >  comments with "Needed for PREEMPT or PREEMPT_LAZY"?
> > > 
> > Good point as well, thank you! And I need to decide whether I should
> > send that patch upstream. Its original purpose was to test PREEMPT_LAZY=y
> > better than could be tested with PREEMPT_LAZY.
> > 
> > Thoughts?
> 
> I will add "Needed for PREEMPT_LAZY" comments, if this commit (935147775c977) is
> send to upstream, will update comments again in the future.

That sounds good to me, thank you!

							Thanx, Paul

> Thanks
> Zqiang
> 
> > 
> >  Thanx, Paul
> > 
> > > 
> > > Thanks
> > >  Zqiang
> > >  
> > >  
> > >  
> > >  Thanx, Paul
> > >  
> > >  > 
> > >  > thanks,
> > >  > 
> > >  > - Joel
> > >  > 
> > >  > 
> > >  > > cookie = get_state_synchronize_srcu(ssp);
> > >  > > if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
> > >  > > - preempt_enable();
> > >  > > return;
> > >  > > }
> > >  > > WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > >  > > @@ -194,7 +193,6 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > >  > > else if (list_empty(&ssp->srcu_work.entry))
> > >  > > list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> > >  > > }
> > >  > > - preempt_enable();
> > >  > > }
> > >  > > 
> > >  > > /*
> > >  > > -- 
> > >  > > 2.48.1
> > >  > >
> > >  >
> > >
> >