Le Fri, Feb 14, 2025 at 01:10:43AM -0800, Paul E. McKenney a écrit : > On Fri, Feb 14, 2025 at 12:25:57AM +0100, Frederic Weisbecker wrote: > > When a grace period is started, the ->expmask of each node is set up > > from sync_exp_reset_tree(). Then later on each leaf node also initialize > > its ->exp_tasks pointer. > > > > This means that the initialization of the quiescent state of a node and > > the initialization of its blocking tasks happen with an unlocked node > > gap in-between. > > > > It happens to be fine because nothing is expected to report an exp > > quiescent state within this gap, since no IPI have been issued yet and > > every rdp's ->cpu_no_qs.b.exp should be false. > > > > However if it were to happen by accident, the quiescent state could be > > reported and propagated while ignoring tasks that blocked _before_ the > > start of the grace period. > > > > Prevent such trouble to happen in the future and initialize both the > > quiescent states mask to report and the blocked tasks head from the same > > node locked block. > > > > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> > > Thank you for looking into this! > > One question: What happens if a CPU has tasks pending during the > call to sync_exp_reset_tree(), but all of these tasks complete > their RCU read-side critical sections before execution reaches > __sync_rcu_exp_select_node_cpus()? > > (My guess is that all is well, but even if so, it would be good to record > why in the commit log.) All is (expected to be) well because the QS won't be reported yet: rdp->cpu_no_qs.b.exp is still false, therefore rnp->expmask will still have the RDPs masks set. !PREEMPT_RCU is different because sync_sched_exp_online_cleanup() can report the QS earlier. But that patch is a PREEMPT_RCU concern only. I'll drop a note on the changelog. Thanks. > > Thanx, Paul > > > --- > > kernel/rcu/tree_exp.h | 14 +++++++------- > > 1 file changed, 7 insertions(+), 7 deletions(-) > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 8d4895c854c5..caff16e441d1 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -141,6 +141,13 @@ static void __maybe_unused sync_exp_reset_tree(void) > > raw_spin_lock_irqsave_rcu_node(rnp, flags); > > WARN_ON_ONCE(rnp->expmask); > > WRITE_ONCE(rnp->expmask, rnp->expmaskinit); > > + /* > > + * Need to wait for any blocked tasks as well. Note that > > + * additional blocking tasks will also block the expedited GP > > + * until such time as the ->expmask bits are cleared. > > + */ > > + if (rcu_is_leaf_node(rnp) && rcu_preempt_has_tasks(rnp)) > > + WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next); > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > } > > } > > @@ -393,13 +400,6 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp) > > } > > mask_ofl_ipi = rnp->expmask & ~mask_ofl_test; > > > > - /* > > - * Need to wait for any blocked tasks as well. Note that > > - * additional blocking tasks will also block the expedited GP > > - * until such time as the ->expmask bits are cleared. > > - */ > > - if (rcu_preempt_has_tasks(rnp)) > > - WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next); > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > > /* IPI the remaining CPUs for expedited quiescent state. */ > > -- > > 2.46.0 > >
