rcu: Don't deboost before reporting expedited quiescent state

commit 10c535787436d62ea28156a4b91365fd89b5a432 upstream.

Currently rcu_preempt_deferred_qs_irqrestore() releases rnp->boost_mtx
before reporting the expedited quiescent state.  Under heavy real-time
load, this can result in this function being preempted before the
quiescent state is reported, which can in turn prevent the expedited grace
period from completing.  Tim Murray reports that the resulting expedited
grace periods can take hundreds of milliseconds and even more than one
second, when they should normally complete in less than a millisecond.

This was fine given that there were no particular response-time
constraints for synchronize_rcu_expedited(), as it was designed
for throughput rather than latency.  However, some users now need
sub-100-millisecond response-time constratints.

This patch therefore follows Neeraj's suggestion (seconded by Tim and
by Uladzislau Rezki) of simply reversing the two operations.

Reported-by: Tim Murray <timmurray@google.com>
Reported-by: Joel Fernandes <joelaf@google.com>
Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Tested-by: Tim Murray <timmurray@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Sandeep Patil <sspatil@google.com>
Cc: <stable@vger.kernel.org> # 5.4.x
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a71a4a272515de2aad0fb297bef81b16a7b34ea8..2c127d438fe0a2422e8b0c673086d2e6a27e6a98 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -523,16 +523,17 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
                        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                }
 
-               /* Unboost if we were boosted. */
-               if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
-                       rt_mutex_futex_unlock(&rnp->boost_mtx);
-
                /*
                 * If this was the last task on the expedited lists,
                 * then we need to report up the rcu_node hierarchy.
                 */
                if (!empty_exp && empty_exp_now)
                        rcu_report_exp_rnp(rnp, true);
+
+               /* Unboost if we were boosted. */
+               if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
+                       rt_mutex_futex_unlock(&rnp->boost_mtx);
+
        } else {
                local_irq_restore(flags);
        }