hrtimer: Annotate lockless access to timer->base

Followup to commit dd2261ed45aa ("hrtimer: Protect lockless access
to timer->base")

lock_hrtimer_base() fetches timer->base without lock exclusion.

Compiler is allowed to read timer->base twice (even if considered dumb)
which could end up trying to lock migration_base and return
&migration_base.

  base = timer->base;
  if (likely(base != &migration_base)) {

       /* compiler reads timer->base again, and now (base == &migration_base)

       raw_spin_lock_irqsave(&base->cpu_base->lock, *flags);
       if (likely(base == timer->base))
            return base; /* == &migration_base ! */

Similarly the write sides must use WRITE_ONCE() to avoid store tearing.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191008173204.180879-1-edumazet@google.com

1 files changed, 4 insertions, 4 deletions

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 0d4dc241c0fb..65605530ee34 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -164,7 +164,7 @@ struct hrtimer_clock_base *lock_hrtimer_base(const struct hrtimer *timer,
         struct hrtimer_clock_base *base;
 
         for (;;) {
-                base = timer->base;
+                base = READ_ONCE(timer->base);
                 if (likely(base != &migration_base)) {
                         raw_spin_lock_irqsave(&base->cpu_base->lock, *flags);
                         if (likely(base == timer->base))
@@ -244,7 +244,7 @@ again:
                         return base;
 
                 /* See the comment in lock_hrtimer_base() */
-                timer->base = &migration_base;
+                WRITE_ONCE(timer->base, &migration_base);
                 raw_spin_unlock(&base->cpu_base->lock);
                 raw_spin_lock(&new_base->cpu_base->lock);
 
@@ -253,10 +253,10 @@ again:
                         raw_spin_unlock(&new_base->cpu_base->lock);
                         raw_spin_lock(&base->cpu_base->lock);
                         new_cpu_base = this_cpu_base;
-                        timer->base = base;
+                        WRITE_ONCE(timer->base, base);
                         goto again;
                 }
-                timer->base = new_base;
+                WRITE_ONCE(timer->base, new_base);
         } else {
                 if (new_cpu_base != this_cpu_base &&
                     hrtimer_check_target(timer, new_base)) {