Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()

[Lai Jiangshan]

On 2019/11/4 10:55 äå, Paul E. McKenney wrote:
> On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote:
> > On 2019/11/3 10:01 äå, Boqun Feng wrote:
> > > Hi Jiangshan,
> > >
> > >
> > > I haven't checked the correctness of this patch carefully, but..
> > >
> > >
> > > On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote:
> > > > Don't need to set ->rcu_read_lock_nesting negative, irq-protected
> > > > rcu_preempt_deferred_qs_irqrestore() doesn't expect
> > > > ->rcu_read_lock_nesting to be negative to work, it even
> > > > doesn't access to ->rcu_read_lock_nesting any more.
> > >
> > > rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
> > > eventually call swake_up() or its friends to wake up, say, the gp
> > > kthread, and the wake up functions could go into the scheduler code
> > > path which might have RCU read-side critical section in it, IOW,
> > > accessing ->rcu_read_lock_nesting.
> >
> > Sure, thank you for pointing it out.
> >
> > I should rewrite the changelog in next round. Like this:
> >
> > rcu: cleanup rcu_preempt_deferred_qs()
> >
> > IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
> > expect ->rcu_read_lock_nesting to be negative to work.
> >
> > There might be RCU read-side critical section in it (from wakeup()
> > or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
> > will ensure that ->rcu_read_unlock_special is zero and these RCU
> > read-side critical sections will not call rcu_read_unlock_special().
> >
> > Thanks
> > Lai
> >
> > ===
> > PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
> > not applied earlier, it will be protected by previous patch (patch1)
> > in this series
> > "rcu: use preempt_count to test whether scheduler locks is held"
> > when rcu_read_unlock_special() is called.
>
> This one in -rcu, you mean?
>
> 5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once")


Yes, but the commit ID is floating in the tree.

> Some adjustment was needed due to my not applying the earlier patches
> that assumed nested interrupts.  Please let me know if further adjustments
> are needed.

I don't think the earlier patches are needed. If the possible? nested
interrupts described in my previous emails is an issue, the patch
"rcu: don't use negative ->rcu_read_lock_nesting" in this
series is enough to fixed it. If any adjustments needed for
this series, I will just put the adjustments the series.

Thanks
Lai

> 							Thanx, Paul
>
> > > Again, haven't checked closely, but this argument in the commit log
> > > seems untrue.
> > >
> > > Regards,
> > > Boqun
> > >
> > > > It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
> > > > may access to ->rcu_read_lock_nesting, but it is still safe
> > > > since rcu_read_unlock_special() can protect itself from NMI.
> > > >
> > > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >    kernel/rcu/tree_plugin.h | 5 -----
> > > >    1 file changed, 5 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > > index aba5896d67e3..2fab8be2061f 100644
> > > > --- a/kernel/rcu/tree_plugin.h
> > > > +++ b/kernel/rcu/tree_plugin.h
> > > > @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > >    static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > >    {
> > > >    	unsigned long flags;
> > > > -	bool couldrecurse = t->rcu_read_lock_nesting >= 0;
> > > >    	if (!rcu_preempt_need_deferred_qs(t))
> > > >    		return;
> > > > -	if (couldrecurse)
> > > > -		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
> > > >    	local_irq_save(flags);
> > > >    	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > > -	if (couldrecurse)
> > > > -		t->rcu_read_lock_nesting += RCU_NEST_BIAS;
> > > >    }
> > > >    /*
> > > > --
> > > > 2.20.1