linux-kernel - [tip: timers/urgent] hrtimer: Annotate lockless access to timer->base

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Date:   Mon, 14 Oct 2019 13:58:50 -0000
From:   "tip-bot2 for Eric Dumazet" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/urgent] hrtimer: Annotate lockless access to timer->base

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     ff229eee3d897f52bd001c841f2d3cce8853ecdc
Gitweb:        https://git.kernel.org/tip/ff229eee3d897f52bd001c841f2d3cce8853ecdc
Author:        Eric Dumazet <edumazet@...gle.com>
AuthorDate:    Tue, 08 Oct 2019 10:32:04 -07:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Mon, 14 Oct 2019 15:51:49 +02:00

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

---
 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 0d4dc24..6560553 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)) {