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

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Message-ID: <63d537e65b7f4b6f77a16112a871103d07a248d2@linux.dev>
Date: Mon, 08 Sep 2025 13:19:44 +0000
From: "Zqiang" <qiang.zhang@...ux.dev>
To: paulmck@...nel.org
Cc: frederic@...nel.org, neeraj.upadhyay@...nel.org, joelagnelf@...dia.com,
 boqun.feng@...il.com, urezki@...il.com, rcu@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] srcu/tiny: Remove preempt_disable/enable() in
 srcu_gp_start_if_needed()

> 
> On Mon, Sep 08, 2025 at 08:31:55AM +0800, Zqiang wrote:
> 
> > 
> > Currently, the srcu_gp_start_if_needed() is always be invoked in
> >  preempt disable's critical section, this commit therefore remove
> >  redundant preempt_disable/enable() in srcu_gp_start_if_needed().
> >  
> >  Fixes: 65b4a59557f6 ("srcu: Make Tiny SRCU explicitly disable preemption")
> >  Signed-off-by: Zqiang <qiang.zhang@...ux.dev>
> > 
> Looks good, but what would be a good way to make this code defend itself
> against being invoked from someplace else that did have preemption
> enabled? Especially given that the Tree SRCU version of this function
> does get invoked with preemption enabled?

ok, maybe we can add lockdep_assert_preemption_disabled() in
the srcu_gp_start_if_needed() ?

Thanks
Zqiang


> 
>  Thanx, Paul
> 
> > 
> > ---
> >  kernel/rcu/srcutiny.c | 3 ---
> >  1 file changed, 3 deletions(-)
> >  
> >  diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> >  index b52ec45698e8..417bd0e4457c 100644
> >  --- a/kernel/rcu/srcutiny.c
> >  +++ b/kernel/rcu/srcutiny.c
> >  @@ -181,10 +181,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> >  {
> >  unsigned long cookie;
> >  
> >  - preempt_disable(); // Needed for PREEMPT_LAZY
> >  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 +192,6 @@ 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
> >
>