On Wed, Jul 20, 2022 at 06:04:55PM -0700, Paul E. McKenney wrote: > On Wed, Jul 20, 2022 at 05:53:38PM -0700, Boqun Feng wrote: > > Hi Paul, > > > > On Mon, Jun 20, 2022 at 03:51:20PM -0700, Paul E. McKenney wrote: > > > This commit switches the existing polled grace-period APIs to use a > > > new ->gp_seq_polled counter in the rcu_state structure. An additional > > > ->gp_seq_polled_snap counter in that same structure allows the normal > > > grace period kthread to interact properly with the !SMP !PREEMPT fastpath > > > through synchronize_rcu(). The first of the two to note the end of a > > > given grace period will make knowledge of this transition available to > > > the polled API. > > > > > > This commit is in preparation for polled expedited grace periods. > > > > > > Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@xxxxxxxxxx/ > > > Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing > > > Cc: Brian Foster <bfoster@xxxxxxxxxx> > > > Cc: Dave Chinner <david@xxxxxxxxxxxxx> > > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > > Cc: Ian Kent <raven@xxxxxxxxxx> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > --- > > > kernel/rcu/tree.c | 90 +++++++++++++++++++++++++++++++++++++++++++++-- > > > kernel/rcu/tree.h | 2 ++ > > > 2 files changed, 89 insertions(+), 3 deletions(-) > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 46cfceea87847..637e8f9454573 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -1775,6 +1775,78 @@ static void rcu_strict_gp_boundary(void *unused) > > > invoke_rcu_core(); > > > } > > > > > > +// Has rcu_init() been invoked? This is used (for example) to determine > > > +// whether spinlocks may be acquired safely. > > > +static bool rcu_init_invoked(void) > > > +{ > > > + return !!rcu_state.n_online_cpus; > > > +} > > > + > > > +// Make the polled API aware of the beginning of a grace period. > > > +static void rcu_poll_gp_seq_start(unsigned long *snap) > > > +{ > > > + struct rcu_node *rnp = rcu_get_root(); > > > + > > > + if (rcu_init_invoked()) > > > + raw_lockdep_assert_held_rcu_node(rnp); > > > + > > > + // If RCU was idle, note beginning of GP. > > > + if (!rcu_seq_state(rcu_state.gp_seq_polled)) > > > + rcu_seq_start(&rcu_state.gp_seq_polled); > > > + > > > + // Either way, record current state. > > > + *snap = rcu_state.gp_seq_polled; > > > +} > > > + > > > +// Make the polled API aware of the end of a grace period. > > > +static void rcu_poll_gp_seq_end(unsigned long *snap) > > > +{ > > > + struct rcu_node *rnp = rcu_get_root(); > > > + > > > + if (rcu_init_invoked()) > > > + raw_lockdep_assert_held_rcu_node(rnp); > > > + > > > + // If the the previously noted GP is still in effect, record the > > > + // end of that GP. Either way, zero counter to avoid counter-wrap > > > + // problems. > > > + if (*snap && *snap == rcu_state.gp_seq_polled) { > > > + rcu_seq_end(&rcu_state.gp_seq_polled); > > > + rcu_state.gp_seq_polled_snap = 0; > > > + } else { > > > + *snap = 0; > > > + } > > > +} > > > + > > > +// Make the polled API aware of the beginning of a grace period, but > > > +// where caller does not hold the root rcu_node structure's lock. > > > +static void rcu_poll_gp_seq_start_unlocked(unsigned long *snap) > > > +{ > > > + struct rcu_node *rnp = rcu_get_root(); > > > + > > > + if (rcu_init_invoked()) { > > > + lockdep_assert_irqs_enabled(); > > > + raw_spin_lock_irq_rcu_node(rnp); > > > + } > > > + rcu_poll_gp_seq_start(snap); > > > + if (rcu_init_invoked()) > > > + raw_spin_unlock_irq_rcu_node(rnp); > > > +} > > > + > > > +// Make the polled API aware of the end of a grace period, but where > > > +// caller does not hold the root rcu_node structure's lock. > > > +static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap) > > > +{ > > > + struct rcu_node *rnp = rcu_get_root(); > > > + > > > + if (rcu_init_invoked()) { > > > + lockdep_assert_irqs_enabled(); > > > + raw_spin_lock_irq_rcu_node(rnp); > > > + } > > > + rcu_poll_gp_seq_end(snap); > > > + if (rcu_init_invoked()) > > > + raw_spin_unlock_irq_rcu_node(rnp); > > > +} > > > + > > > /* > > > * Initialize a new grace period. Return false if no grace period required. > > > */ > > > @@ -1810,6 +1882,7 @@ static noinline_for_stack bool rcu_gp_init(void) > > > rcu_seq_start(&rcu_state.gp_seq); > > > ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq); > > > trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start")); > > > + rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap); > > > raw_spin_unlock_irq_rcu_node(rnp); > > > > > > /* > > > @@ -2069,6 +2142,7 @@ static noinline void rcu_gp_cleanup(void) > > > * safe for us to drop the lock in order to mark the grace > > > * period as completed in all of the rcu_node structures. > > > */ > > > + rcu_poll_gp_seq_end(&rcu_state.gp_seq_polled_snap); > > > raw_spin_unlock_irq_rcu_node(rnp); > > > > > > /* > > > @@ -3837,8 +3911,18 @@ void synchronize_rcu(void) > > > lock_is_held(&rcu_lock_map) || > > > lock_is_held(&rcu_sched_lock_map), > > > "Illegal synchronize_rcu() in RCU read-side critical section"); > > > - if (rcu_blocking_is_gp()) > > > + if (rcu_blocking_is_gp()) { > > > + // Note well that this code runs with !PREEMPT && !SMP. > > > + // In addition, all code that advances grace periods runs > > > + // at process level. Therefore, this GP overlaps with other > > > + // GPs only by being fully nested within them, which allows > > > + // reuse of ->gp_seq_polled_snap. > > > + rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_snap); > > > + rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_snap); > > > + if (rcu_init_invoked()) > > > + cond_resched_tasks_rcu_qs(); > > > return; // Context allows vacuous grace periods. > > > + } > > > if (rcu_gp_is_expedited()) > > > synchronize_rcu_expedited(); > > > else > > > @@ -3860,7 +3944,7 @@ unsigned long get_state_synchronize_rcu(void) > > > * before the load from ->gp_seq. > > > */ > > > smp_mb(); /* ^^^ */ > > > - return rcu_seq_snap(&rcu_state.gp_seq); > > > + return rcu_seq_snap(&rcu_state.gp_seq_polled); > > > > I happened to run into this. There is one usage of > > get_state_synchronize_rcu() in start_poll_synchronize_rcu(), in which > > the return value of get_state_synchronize_rcu() ("gp_seq") will be used > > for rcu_start_this_gp(). I don't think this is quite right, because > > after this change, rcu_state.gp_seq and rcu_state.gp_seq_polled are > > different values, in fact ->gp_seq_polled is greater than ->gp_seq > > by how many synchronize_rcu() is called in early boot. > > > > Am I missing something here? > > It does not appear that your are missing anything, sad to say! > > Does the following make it work better? Well, rcutorture doesn't like this change much. ;-) No surprise, given that it is only the value feeding into rcu_start_this_gp() that needs to change, not the value returned from start_poll_synchronize_rcu(). Take 2, still untested. Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2122359f0c862..061c1f6737ddc 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3581,7 +3581,7 @@ unsigned long start_poll_synchronize_rcu(void) rdp = this_cpu_ptr(&rcu_data); rnp = rdp->mynode; raw_spin_lock_rcu_node(rnp); // irqs already disabled. - needwake = rcu_start_this_gp(rnp, rdp, gp_seq); + needwake = rcu_start_this_gp(rnp, rdp, rcu_seq_snap(&rcu_state.gp_seq)); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); if (needwake) rcu_gp_kthread_wake();
