rcu: Fix RCU CPU stall detection in tiny implementation

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@suse.cz>
[ paulmck: Fused identical "if" statements, ported to -rcu. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
index 858c56569127..80f908afe348 100644
--- a/kernel/rcu/tiny_plugin.h
+++ b/kernel/rcu/tiny_plugin.h
@@ -145,17 +145,16 @@ 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))
                 ACCESS_ONCE(rcp->jiffies_stall) = jiffies +
                         3 * rcu_jiffies_till_stall_check() + 3;
-        else if (ULONG_CMP_GE(j, js))
+        } else if (ULONG_CMP_GE(j, js)) {
                 ACCESS_ONCE(rcp->jiffies_stall) = jiffies + rcu_jiffies_till_stall_check();
+        }
 }
 
 static void reset_cpu_stall_ticks(struct rcu_ctrlblk *rcp)