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

[Lai Jiangshan]

On 2019/11/11 10:32 äå, Paul E. McKenney wrote:
> On Mon, Nov 04, 2019 at 11:19:11PM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 05, 2019 at 10:09:15AM +0800, Lai Jiangshan wrote:
> > > 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.
> >
> > Indeed, that part of -rcu is subject to rebase, and will continue
> > to be until about v5.5-rc5 or thereabouts.
> >
> > https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/rcutodo.html
> >
> > My testing of your full stack should be complete by this coming Sunday
> > morning, Pacific Time.
>
> And you will be happy to hear that it ran the full time without errors.
>
> Good show!!!
>
> My next step is to look much more carefully at the remaining patches,
> checking my first impressions.  This will take a few days.

Hi, All

I'm still asking for more comments.

By now, I have received some precious comments, mainly due to my
stupid naming mistakes and a misleading changelog. I should have
updated all these with a new series patches. But I hope I
can polish more in the new patchset with more suggestions from
valuable comments, especially in x86,scheduler,percpu and rcu
areas.

I'm very obliged to hear anything.

Thanks
Lai