sched/nohz: Fix overflow error in scheduler_tick_max_deferment()

While calculating the scheduler tick max deferment, the delta is
converted from microseconds to nanoseconds through a multiplication
against NSEC_PER_USEC.

But this microseconds operand is an unsigned int, thus the result may
likely overflow. The result is cast to u64 but only once the operation
is completed, which is too late to avoid overflown result.

This is currently not a problem because the scheduler tick max deferment
is 1 second. But this may become an issue as we plan to make this
value tunable.

So lets fix this by casting the usecs value to u64 before multiplying by
NSECS_PER_USEC.

Also to prevent from this kind of mistake to happen again, move this
ad-hoc jiffies -> nsecs conversion to a new helper.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alex Shi <alex.shi@linaro.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Kevin Hilman <khilman@linaro.org>
Link: http://lkml.kernel.org/r/1387315388-31676-2-git-send-email-khilman@linaro.org
[move ad-hoc conversion to jiffies_to_nsecs helper]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index d235e88cfd7c41c9998d094dc24b6f0f46ded937..1f44466c1e9d7b056cbdcd4f7c043018f069fad4 100644 (file)
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -294,6 +294,12 @@ extern unsigned long preset_lpj;
  */
 extern unsigned int jiffies_to_msecs(const unsigned long j);
 extern unsigned int jiffies_to_usecs(const unsigned long j);
+
+static inline u64 jiffies_to_nsecs(const unsigned long j)
+{
+       return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC;
+}
+
 extern unsigned long msecs_to_jiffies(const unsigned int m);
 extern unsigned long usecs_to_jiffies(const unsigned int u);
 extern unsigned long timespec_to_jiffies(const struct timespec *value);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a88f4a485c5e5f92190dd5bf784600d79d5f8f18..61e601fc2b1e5ae98f9b5f02c0ccd7fe096fd3e4 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2325,7 +2325,7 @@ u64 scheduler_tick_max_deferment(void)
        if (time_before_eq(next, now))
                return 0;
 
-       return jiffies_to_usecs(next - now) * NSEC_PER_USEC;
+       return jiffies_to_nsecs(next - now);
 }
 #endif