[PATCH] rcu: Fix RCU CPU stall detection in tiny implementation

[Miroslav Benes]

The tiny RCU CPU stall detection depends on *rcp->curtail not being
NULL. It is however a tail pointer and thus NULL by definition. Instead we
should check rcp->rcucblist for the presence of pending callbacks which need to
be processed. With this fix INFO about the stall is printed and jiffies_stall
(jiffies at next stall) correctly updated.

Note that the check for pending callback is necessary to avoid spurious warnings
if there are no pendings callbacks.

Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
---
 kernel/rcu/tiny_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
index 858c565..868e132 100644
--- a/kernel/rcu/tiny_plugin.h
+++ b/kernel/rcu/tiny_plugin.h
@@ -145,13 +145,13 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
 	rcp->ticks_this_gp++;
 	j = jiffies;
 	js = ACCESS_ONCE(rcp->jiffies_stall);
-	if (*rcp->curtail && ULONG_CMP_GE(j, js)) {
+	if (rcp->rcucblist && ULONG_CMP_GE(j, js)) {
 		pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)\n",
 		       rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting,
 		       jiffies - rcp->gp_start, rcp->qlen);
 		dump_stack();
 	}
-	if (*rcp->curtail && ULONG_CMP_GE(j, js))
+	if (rcp->rcucblist && ULONG_CMP_GE(j, js))
 		ACCESS_ONCE(rcp->jiffies_stall) = jiffies +
 			3 * rcu_jiffies_till_stall_check() + 3;
 	else if (ULONG_CMP_GE(j, js))
-- 
2.1.2

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