Re: [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code entrypoints

[Joel Fernandes]

On Wed, May 13, 2020 at 06:47:05PM +0200, Frederic Weisbecker wrote:
> Pure NOCB code entrypoints (nocb_cb kthread, nocb_gp kthread, nocb
> timers) can unconditionally lock rdp->nocb_lock as they always execute
> in the context of an offloaded rdp.
> 
> This also prepare for toggling CPUs to/from callback's offloaded mode
> where the offloaded state will possibly change when rdp->nocb_lock
> isn't taken. We'll still want the entrypoints to lock the rdp in any
> case.

Suggested rewrite for change log:
<wordsmithing>
Make pure NOCB code entrypoints (nocb_cb kthread, nocb_gp kthread, nocb
timers) unconditionally lock rdp->nocb_lock as they always execute in the
context of an offloaded rdp.

This prepares for future toggling of CPUs to/from callback's offloaded mode
where the offloaded state can change when rdp->nocb_lock is not held. We'll
still want the entrypoints to lock the rdp in any case.
</wordsmithing>

Also, can we inline rcu_nocb_lock_irqsave() into
do_nocb_deferred_wakeup_common() since that's the only user, and then delete
rcu_nocb_lock_irqsave() and the corresponding unlock? That would also remove
confusion about which API to use for nocb locking (i.e. whether to directly
acquire lock or call rcu_nocb_lock_irqsave()).

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel


> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree_plugin.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 097635c41135..523570469864 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1909,7 +1909,7 @@ static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
>  	struct rcu_data *rdp = from_timer(rdp, t, nocb_bypass_timer);
>  
>  	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
> -	rcu_nocb_lock_irqsave(rdp, flags);
> +	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
>  	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
>  	__call_rcu_nocb_wake(rdp, true, flags);
>  }
> @@ -1942,7 +1942,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  	 */
>  	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
>  		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
> -		rcu_nocb_lock_irqsave(rdp, flags);
> +		raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
>  		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
>  		if (bypass_ncbs &&
>  		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
> @@ -1951,7 +1951,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  			(void)rcu_nocb_try_flush_bypass(rdp, j);
>  			bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
>  		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
> -			rcu_nocb_unlock_irqrestore(rdp, flags);
> +			raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
>  			continue; /* No callbacks here, try next. */
>  		}
>  		if (bypass_ncbs) {
> @@ -1996,7 +1996,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  		} else {
>  			needwake = false;
>  		}
> -		rcu_nocb_unlock_irqrestore(rdp, flags);
> +		raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
>  		if (needwake) {
>  			swake_up_one(&rdp->nocb_cb_wq);
>  			gotcbs = true;
> @@ -2084,7 +2084,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  	rcu_do_batch(rdp);
>  	local_bh_enable();
>  	lockdep_assert_irqs_enabled();
> -	rcu_nocb_lock_irqsave(rdp, flags);
> +	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
>  	if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
>  	    rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
>  	    raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
> @@ -2092,7 +2092,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
>  	}
>  	if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
> -		rcu_nocb_unlock_irqrestore(rdp, flags);
> +		raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
>  		if (needwake_gp)
>  			rcu_gp_kthread_wake();
>  		return;
> @@ -2100,7 +2100,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  
>  	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
>  	WRITE_ONCE(rdp->nocb_cb_sleep, true);
> -	rcu_nocb_unlock_irqrestore(rdp, flags);
> +	raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
>  	if (needwake_gp)
>  		rcu_gp_kthread_wake();
>  	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> -- 
> 2.25.0
>