rcu: Fix CONFIG_RCU_FAST_NO_HZ stall warning message

The print_cpu_stall_fast_no_hz() function attempts to print -1 when
the ->idle_gp_timer is not pending, but unsigned arithmetic causes it
to instead print ULONG_MAX, which is 4294967295 on 32-bit systems and
18446744073709551615 on 64-bit systems.  Neither of these are the most
reader-friendly values, so this commit instead causes "timer not pending"
to be printed when ->idle_gp_timer is not pending.

Reported-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

2 files changed, 16 insertions, 12 deletions

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 523364e4e1f1..1927151b386b 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -99,7 +99,7 @@ In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
 printed:
 
         INFO: rcu_preempt detected stall on CPU
-        0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer=-1
+        0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer not pending
            (t=65000 jiffies)
 
 The "(64628 ticks this GP)" indicates that this CPU has taken more
@@ -116,13 +116,13 @@ number between the two "/"s is the value of the nesting, which will
 be a small positive number if in the idle loop and a very large positive
 number (as shown above) otherwise.
 
-For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the
-CPU is not in the process of trying to force itself into dyntick-idle
-state, the "." indicates that the CPU has not given up forcing RCU
-into dyntick-idle mode (it would be "H" otherwise), and the "timer=-1"
-indicates that the CPU has not recented forced RCU into dyntick-idle
-mode (it would otherwise indicate the number of microseconds remaining
-in this forced state).
+For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the CPU is
+not in the process of trying to force itself into dyntick-idle state, the
+"." indicates that the CPU has not given up forcing RCU into dyntick-idle
+mode (it would be "H" otherwise), and the "timer not pending" indicates
+that the CPU has not recently forced RCU into dyntick-idle mode (it
+would otherwise indicate the number of microseconds remaining in this
+forced state).
 
 
 Multiple Warnings From One Stall
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index df47014e129d..e12d07ba601a 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2130,11 +2130,15 @@ static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
 {
         struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
         struct timer_list *tltp = &rdtp->idle_gp_timer;
+        char c;
 
-        sprintf(cp, "drain=%d %c timer=%lu",
-                rdtp->dyntick_drain,
-                rdtp->dyntick_holdoff == jiffies ? 'H' : '.',
-                timer_pending(tltp) ? tltp->expires - jiffies : -1);
+        c = rdtp->dyntick_holdoff == jiffies ? 'H' : '.';
+        if (timer_pending(tltp))
+                sprintf(cp, "drain=%d %c timer=%lu",
+                        rdtp->dyntick_drain, c, tltp->expires - jiffies);
+        else
+                sprintf(cp, "drain=%d %c timer not pending",
+                        rdtp->dyntick_drain, c);
 }
 
 #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */