Re: [PATCH v2] srcu/tiny: Remove preempt_disable/enable() in srcu_gp_start_if_needed()

["Zqiang" <qiang.zhang@linux.dev>]

> 
> 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.

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
> >  > >
> >  >
> >
>