[PATCH] hrtimer: annotate lockless access to timer->base

[Eric Dumazet]

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)
and we 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 should use WRITE_ONCE() to avoid
store tearing.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Julien Grall <julien.gr...@arm.com>
Cc: Thomas Gleixner <t...@linutronix.de>
---
 kernel/time/hrtimer.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 
0d4dc241c0fb498036c91a571e65cb00f5d19ba6..65605530ee349c9682690c4fccb43aa9284d4287
 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 @@ switch_hrtimer_base(struct hrtimer *timer, struct 
hrtimer_clock_base *base,
                        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 @@ switch_hrtimer_base(struct hrtimer *timer, struct 
hrtimer_clock_base *base,
                        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)) {
-- 
2.23.0.581.g78d2f28ef7-goog