[PATCH 3/9] rcu: Don't panic on already-ended RCU CPU stalls

["Paul E. McKenney" <paulmck@kernel.org>]

The current RCU CPU stall-warning code can say "INFO: Stall ended before
state dump start" but then panic the system anyway.  This commit therefore
avoids panicking the system if the stall has already ended.

However, the sched_ext scheduler will still be ejected, as it might have
been the cause of the almost stall.  This can easily be changed if needed.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_exp.h   |  2 +-
 kernel/rcu/tree_stall.h | 12 +++++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 81d42b6e2e31..01bdbd7e28cc 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -678,7 +678,7 @@ static void synchronize_rcu_expedited_wait(void)
 
 		nbcon_cpu_emergency_exit();
 
-		panic_on_rcu_stall(&rcu_exp_stall_cpumask);
+		panic_on_rcu_stall(&rcu_exp_stall_cpumask, NULL, 0);
 	}
 }
 
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 091e7850ab6e..93ba31a619b6 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -159,7 +159,8 @@ static int __init check_cpu_stall_init(void)
 early_initcall(check_cpu_stall_init);
 
 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
-static void panic_on_rcu_stall(const struct cpumask *stalled_mask)
+static void panic_on_rcu_stall(const struct cpumask *stalled_mask,
+			       unsigned long *gsp, unsigned long gp_seq)
 {
 	static int cpu_stall;
 
@@ -170,6 +171,11 @@ static void panic_on_rcu_stall(const struct cpumask *stalled_mask)
 	if (scx_rcu_cpu_stall(stalled_mask))
 		return;
 
+	if (gsp && rcu_seq_current(gsp) != gp_seq) {
+		pr_err("INFO: Stall ended before panic check.\n");
+		return;
+	}
+
 	if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
 		return;
 
@@ -703,7 +709,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 
 	nbcon_cpu_emergency_exit();
 
-	panic_on_rcu_stall(&rcu_stall_cpumask);
+	panic_on_rcu_stall(&rcu_stall_cpumask, &rcu_state.gp_seq, gp_seq);
 
 	rcu_force_quiescent_state();  /* Kick them all. */
 }
@@ -758,7 +764,7 @@ static void print_cpu_stall(unsigned long gp_seq, unsigned long gps)
 
 	cpumask_clear(&rcu_stall_cpumask);
 	cpumask_set_cpu(smp_processor_id(), &rcu_stall_cpumask);
-	panic_on_rcu_stall(&rcu_stall_cpumask);
+	panic_on_rcu_stall(&rcu_stall_cpumask, &rcu_state.gp_seq, gp_seq);
 
 	/*
 	 * Attempt to revive the RCU machinery by forcing a context switch.
-- 
2.40.1