1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index c685e31492df..c3ae1446461c 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -176,10 +176,36 @@ static u64 sched_clock_remote(struct sched_clock_data *scd)
         u64 this_clock, remote_clock;
         u64 *ptr, old_val, val;
 
+#if BITS_PER_LONG != 64
+again:
+        /*
+         * Careful here: The local and the remote clock values need to
+         * be read out atomic as we need to compare the values and
+         * then update either the local or the remote side. So the
+         * cmpxchg64 below only protects one readout.
+         *
+         * We must reread via sched_clock_local() in the retry case on
+         * 32bit as an NMI could use sched_clock_local() via the
+         * tracer and hit between the readout of
+         * the low32bit and the high 32bit portion.
+         */
+        this_clock = sched_clock_local(my_scd);
+        /*
+         * We must enforce atomic readout on 32bit, otherwise the
+         * update on the remote cpu can hit inbetween the readout of
+         * the low32bit and the high 32bit portion.
+         */
+        remote_clock = cmpxchg64(&scd->clock, 0, 0);
+#else
+        /*
+         * On 64bit the read of [my]scd->clock is atomic versus the
+         * update, so we can avoid the above 32bit dance.
+         */
         sched_clock_local(my_scd);
 again:
         this_clock = my_scd->clock;
         remote_clock = scd->clock;
+#endif
 
         /*
          * Use the opportunity that we have both locks