@@ -842,28 +842,24 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
struct rcu_node *rnp = rcu_get_root(rsp);
if (!rcu_scheduler_fully_active ||
- !cpu_needs_another_gp(rsp, rdp) ||
- rsp->fqs_active) {
- if (rcu_scheduler_fully_active &&
- cpu_needs_another_gp(rsp, rdp))
- rsp->fqs_need_gp = 1;
- if (rnp->completed == rsp->completed) {
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- return;
- }
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ !cpu_needs_another_gp(rsp, rdp)) {
+ /*
+ * Either the scheduler hasn't yet spawned the first
+ * non-idle task or this CPU does not need another
+ * grace period. Either way, don't start a new grace
+ * period.
+ */
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ return;
+ }
+ if (rsp->fqs_active) {
/*
- * Propagate new ->completed value to rcu_node structures
- * so that other CPUs don't have to wait until the start
- * of the next grace period to process their callbacks.
+ * This CPU needs a grace period, but force_quiescent_state()
+ * is running. Tell it to start one on this CPU's behalf.
*/
- rcu_for_each_node_breadth_first(rsp, rnp) {
- raw_spin_lock(&rnp->lock); /* irqs already disabled. */
- rnp->completed = rsp->completed;
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
- }
- local_irq_restore(flags);
+ rsp->fqs_need_gp = 1;
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
@@ -947,6 +943,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
__releases(rcu_get_root(rsp)->lock)
{
unsigned long gp_duration;
+ struct rcu_node *rnp = rcu_get_root(rsp);
+ struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
@@ -958,7 +956,40 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
gp_duration = jiffies - rsp->gp_start;
if (gp_duration > rsp->gp_max)
rsp->gp_max = gp_duration;
- rsp->completed = rsp->gpnum;
+
+ /*
+ * We know the grace period is complete, but to everyone else
+ * it appears to still be ongoing. But it is also the case
+ * that to everyone else it looks like there is nothing that
+ * they can do to advance the grace period. It is therefore
+ * safe for us to drop the lock in order to mark the grace
+ * period as completed in all of the rcu_node structures.
+ *
+ * But if this CPU needs another grace period, it will take
+ * care of this while initializing the next grace period.
+ * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
+ * because the callbacks have not yet been advanced: Those
+ * callbacks are waiting on the grace period that just now
+ * completed.
+ */
+ if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+
+ /*
+ * Propagate new ->completed value to rcu_node structures
+ * so that other CPUs don't have to wait until the start
+ * of the next grace period to process their callbacks.
+ */
+ rcu_for_each_node_breadth_first(rsp, rnp) {
+ raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ rnp->completed = rsp->gpnum;
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ }
+ rnp = rcu_get_root(rsp);
+ raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ }
+
+ rsp->completed = rsp->gpnum; /* Declare the grace period complete. */
trace_rcu_grace_period(rsp->name, rsp->completed, "end");
rsp->signaled = RCU_GP_IDLE;
rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
It is possible for the CPU that noted the end of the prior grace period to not need a new one, and therefore to decide to propagate ->completed throughout the rcu_node tree without starting another grace period. However, in so doing, it releases the root rcu_node structure's lock, which can allow some other CPU to start another grace period. The first CPU will be propagating ->completed in parallel with the second CPU initializing the rcu_node tree for the new grace period. In theory this is harmless, but in practice we need to keep things simple. This commit therefore moves the propagation of ->completed to rcu_report_qs_rsp(), and refrains from marking the old grace period as having been completed until it has finished doing this. This prevents anyone from starting a new grace period concurrently with marking the old grace period as having been completed. Of course, the optimization where a CPU needing a new grace period doesn't bother marking the old one completed is still in effect: In that case, the marking happens implicitly as part of initializing the new grace period. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> --- kernel/rcutree.c | 71 ++++++++++++++++++++++++++++++++++++++--------------- 1 files changed, 51 insertions(+), 20 deletions(-)