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rcu: Switch polled grace-period APIs to ->gp_seq_polled

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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.

[ paulmck: Fix use of rcu_state.gp_seq_polled to start normal grace period. ]

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-04-13 17:46:15 -07:00
parent d0eac20f99
commit bf95b2bc3e
2 changed files with 96 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

98
kernel/rcu/tree.c
View File

@ -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 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);
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
@ -3889,7 +3973,13 @@ 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);
// Note it is possible for a grace period to have elapsed between
// the above call to get_state_synchronize_rcu() and the below call
// to rcu_seq_snap. This is OK, the worst that happens is that we
// get a grace period that no one needed. These accesses are ordered
// by smp_mb(), and we are accessing them in the opposite order
// from which they are updated at grace-period start, as required.
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();
@ -3925,7 +4015,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);
bool poll_state_synchronize_rcu(unsigned long oldstate)
{
if (oldstate == RCU_GET_STATE_COMPLETED ||
rcu_seq_done_exact(&rcu_state.gp_seq, oldstate)) {
rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
smp_mb(); /* Ensure GP ends before subsequent accesses. */
return true;
}

2
kernel/rcu/tree.h
View File

@ -323,6 +323,8 @@ struct rcu_state {
short gp_state; /* GP kthread sleep state. */
unsigned long gp_wake_time; /* Last GP kthread wake. */
unsigned long gp_wake_seq; /* ->gp_seq at ^^^. */
unsigned long gp_seq_polled; /* GP seq for polled API. */
unsigned long gp_seq_polled_snap; /* ->gp_seq_polled at normal GP start. */
/* End of fields guarded by root rcu_node's lock. */