linux-kernel - Re: [PATCH rcu 2/6] rcu: Remove superfluous full memory barrier upon first EQS snapshot

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Date: Thu, 27 Jun 2024 00:03:16 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Neeraj upadhyay <neeraj.iitr10@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH rcu 2/6] rcu: Remove superfluous full memory barrier upon
 first EQS snapshot

Le Wed, Jun 26, 2024 at 10:49:05PM +0530, Neeraj upadhyay a écrit :
> On Wed, Jun 26, 2024 at 7:43 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> >
> > Le Wed, Jun 12, 2024 at 01:57:20PM +0530, Neeraj upadhyay a écrit :
> > > On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > >
> > > > From: Frederic Weisbecker <frederic@...nel.org>
> > > >
> > > > When the grace period kthread checks the extended quiescent state
> > > > counter of a CPU, full ordering is necessary to ensure that either:
> > > >
> > > > * If the GP kthread observes the remote target in an extended quiescent
> > > >   state, then that target must observe all accesses prior to the current
> > > >   grace period, including the current grace period sequence number, once
> > > >   it exits that extended quiescent state.
> > > >
> > > > or:
> > > >
> > > > * If the GP kthread observes the remote target NOT in an extended
> > > >   quiescent state, then the target further entering in an extended
> > > >   quiescent state must observe all accesses prior to the current
> > > >   grace period, including the current grace period sequence number, once
> > > >   it enters that extended quiescent state.
> > > >
> > > > This ordering is enforced through a full memory barrier placed right
> > > > before taking the first EQS snapshot. However this is superfluous
> > > > because the snapshot is taken while holding the target's rnp lock which
> > > > provides the necessary ordering through its chain of
> > > > smp_mb__after_unlock_lock().
> > > >
> > > > Remove the needless explicit barrier before the snapshot and put a
> > > > comment about the implicit barrier newly relied upon here.
> > > >
> > > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > ---
> > > >  .../Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst    | 6 +++---
> > > >  kernel/rcu/tree.c                                          | 7 ++++++-
> > > >  2 files changed, 9 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
> > > > index 5750f125361b0..728b1e690c646 100644
> > > > --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
> > > > +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
> > > > @@ -149,9 +149,9 @@ This case is handled by calls to the strongly ordered
> > > >  ``atomic_add_return()`` read-modify-write atomic operation that
> > > >  is invoked within ``rcu_dynticks_eqs_enter()`` at idle-entry
> > > >  time and within ``rcu_dynticks_eqs_exit()`` at idle-exit time.
> > > > -The grace-period kthread invokes ``rcu_dynticks_snap()`` and
> > > > -``rcu_dynticks_in_eqs_since()`` (both of which invoke
> > > > -an ``atomic_add_return()`` of zero) to detect idle CPUs.
> > > > +The grace-period kthread invokes first ``ct_dynticks_cpu_acquire()``
> > > > +(preceded by a full memory barrier) and ``rcu_dynticks_in_eqs_since()``
> > > > +(both of which rely on acquire semantics) to detect idle CPUs.
> > > >
> > > >  +-----------------------------------------------------------------------+
> > > >  | **Quick Quiz**:                                                       |
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index f07b8bff4621b..1a6ef9c5c949e 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -769,7 +769,12 @@ static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
> > > >   */
> > > >  static int dyntick_save_progress_counter(struct rcu_data *rdp)
> > > >  {
> > > > -       rdp->dynticks_snap = rcu_dynticks_snap(rdp->cpu);
> > > > +       /*
> > > > +        * Full ordering against accesses prior current GP and also against
> > > > +        * current GP sequence number is enforced by current rnp locking
> > > > +        * with chained smp_mb__after_unlock_lock().
> > > > +        */
> > >
> > > It might be worth mentioning that this chained smp_mb__after_unlock_lock()
> > > is provided by rnp leaf node locking in rcu_gp_init() and rcu_gp_fqs_loop() ?
> >
> > Right!
> >
> > How about this?
> >
> 
> Looks good to me, thanks! Minor comment (ditto for the other patch) below
> 
> 
> >     /*
> >      * Full ordering against accesses prior current GP and also against
> 
> Nit: "prior to current GP" ?

Thanks. On a second thought and just to make sure we don't forget why we did
what we did after a few years, I expanded some more, still ok with the following?

	/*
	 * Full ordering between remote CPU's post idle accesses and updater's
	 * accesses prior to current GP (and also the started GP sequence number)
	 * is enforced by rcu_seq_start() implicit barrier and even further by
	 * smp_mb__after_unlock_lock() barriers chained all the way throughout the
	 * rnp locking tree since rcu_gp_init() and up to the current leaf rnp
	 * locking.
	 *
	 * Ordering between remote CPU's pre idle accesses and post grace period's
	 * accesses is enforced by the below acquire semantic.
	 */

Thanks.