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Date:   Thu, 5 Mar 2020 15:05:15 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Zhaolong Zhang <zhangzl2013@126.com>
Cc:     Zhaolong Zhang <zhangzl2013@126.com.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: Fix the (t=0 jiffies) false positive
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On Thu, Mar 05, 2020 at 03:45:57PM +0800, Zhaolong Zhang wrote:
> Calculate 't' with the previously recorded 'gps' instead of 'gp_start'.
> 
> Signed-off-by: Zhaolong Zhang <zhangzl2013@126.com>

Good catch, thank you!

I had to apply this by hand.  My guess is that you developed against
mainline rather than the "dev" branch of the -rcu tree:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

Or perhaps your email client corrupted the patch?

Regardless, please check the version below and let me know if I messed
anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit 7dd581bbbcc00246fec35eda60add4b32200b0a0
Author: Zhaolong Zhang <zhangzl2013@126.com>
Date:   Thu Mar 5 14:56:11 2020 -0800

    rcu: Fix the (t=0 jiffies) false positive
    
    It is possible that an over-long grace period will end while the RCU
    CPU stall warning message is printing.  In this case, the estimate of
    the offending grace period's duration can be erroneous due to refetching
    of rcu_state.gp_start, which will now be the time of the newly started
    grace period.  Computation of this duration clearly needs to use the
    start time for the old over-long grace period, not the fresh new one.
    This commit avoids such errors by causing both print_other_cpu_stall() and
    print_cpu_stall() to reuse the value previously fetched by their caller.
    
    Signed-off-by: Zhaolong Zhang <zhangzl2013@126.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b17cd9b..502b4dd 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -371,7 +371,7 @@ static void rcu_check_gp_kthread_starvation(void)
 	}
 }
 
-static void print_other_cpu_stall(unsigned long gp_seq)
+static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 {
 	int cpu;
 	unsigned long flags;
@@ -408,7 +408,7 @@ static void print_other_cpu_stall(unsigned long gp_seq)
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_get_n_cbs_cpu(cpu);
 	pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
-	       smp_processor_id(), (long)(jiffies - rcu_state.gp_start),
+	       smp_processor_id(), (long)(jiffies - gps),
 	       (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
 	if (ndetected) {
 		rcu_dump_cpu_stacks();
@@ -442,7 +442,7 @@ static void print_other_cpu_stall(unsigned long gp_seq)
 	rcu_force_quiescent_state();  /* Kick them all. */
 }
 
-static void print_cpu_stall(void)
+static void print_cpu_stall(unsigned long gps)
 {
 	int cpu;
 	unsigned long flags;
@@ -467,7 +467,7 @@ static void print_cpu_stall(void)
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_get_n_cbs_cpu(cpu);
 	pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
-		jiffies - rcu_state.gp_start,
+		jiffies - gps,
 		(long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
 
 	rcu_check_gp_kthread_starvation();
@@ -546,7 +546,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 
 		/* We haven't checked in, so go dump stack. */
-		print_cpu_stall();
+		print_cpu_stall(gps);
 		if (rcu_cpu_stall_ftrace_dump)
 			rcu_ftrace_dump(DUMP_ALL);
 
@@ -555,7 +555,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
 
 		/* They had a few time units to dump stack, so complain. */
-		print_other_cpu_stall(gs2);
+		print_other_cpu_stall(gs2, gps);
 		if (rcu_cpu_stall_ftrace_dump)
 			rcu_ftrace_dump(DUMP_ALL);
 	}