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

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Message-ID: <70fa9fd07e4fe2b1b498109f3450804e731fd7ce@linux.dev>
Date: Thu, 11 Sep 2025 00:36:45 +0000
From: "Zqiang" <qiang.zhang@...ux.dev>
To: paulmck@...nel.org, "Joel Fernandes" <joelagnelf@...dia.com>
Cc: 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 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"?

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
> >  >
> >
>