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>

diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
index 858c565..80f908a 100644 (file)
--- 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)