linux-kernel - Re: [PATCH 1/3] srcu: Remove superfluous ssp initialization on deferred work queue

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Date:   Thu, 1 Apr 2021 18:19:32 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 1/3] srcu: Remove superfluous ssp initialization on
 deferred work queue

On Fri, Apr 02, 2021 at 02:58:13AM +0200, Frederic Weisbecker wrote:
> On Thu, Apr 01, 2021 at 05:48:56PM -0700, Paul E. McKenney wrote:
> > On Fri, Apr 02, 2021 at 01:47:02AM +0200, Frederic Weisbecker wrote:
> > > When an ssp has already started a grace period and queued an early work
> > > to flush after SRCU workqueues are created, we expect the ssp to be
> > > properly initialized already. So we can skip this step at this stage.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > Cc: Boqun Feng <boqun.feng@...il.com>
> > > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > > Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> > > Cc: Josh Triplett <josh@...htriplett.org>
> > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > Cc: Uladzislau Rezki <urezki@...il.com>
> > > ---
> > >  kernel/rcu/srcutree.c | 1 -
> > >  1 file changed, 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index 036ff5499ad5..7197156418e4 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -1396,7 +1396,6 @@ void __init srcu_init(void)
> > >  	while (!list_empty(&srcu_boot_list)) {
> > >  		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
> > >  				      work.work.entry);
> > > -		check_init_srcu_struct(ssp);
> > 
> > You lost me on this one.  What happens if the only pre-initialization
> > invocation on the statically allocated srcu_struct pointed to by ssp
> > was call_srcu()?  I am not seeing how the initialization has already
> > happened in that case.
> > 
> > What am I missing here?
> 
> call_srcu() -> __call_srcu() -> srcu_gp_start_if_needed() ->
> check_init_srcu_struct() ?
> 
> Or is it me missing something?

Nope, me getting confused between Tree SRCU's and Tiny SRCU's
call_srcu() implementation.  :-/

I have queued this patch and started testing.

							Thanx, Paul