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?
/*
* Full ordering against accesses prior current GP and also against
* 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.
