On Wed, Jun 26, 2024 at 7:43 PM Frederic Weisbecker <frederic@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > > From: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > ---
> > > .../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
Neeraj
> * current 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.
> */
>
> Thanks.
