[PATCH] mailbox: cancel timer before starting it

[Jerome Brunet]

If the txdone is done by polling, it is possible for msg_submit() to start
the timer while txdone_hrtimer() callback is running. If the timer needs
recheduling, it could already be enqueued by the time hrtimer_forward_now()
is called, leading hrtimer to loudly complain.

WARNING: CPU: 3 PID: 74 at kernel/time/hrtimer.c:932 hrtimer_forward+0xc4/0x110
CPU: 3 PID: 74 Comm: kworker/u8:1 Not tainted 5.9.0-rc2-00236-gd3520067d01c-dirty #5
Hardware name: Libre Computer AML-S805X-AC (DT)
Workqueue: events_freezable_power_ thermal_zone_device_check
pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
pc : hrtimer_forward+0xc4/0x110
lr : txdone_hrtimer+0xf8/0x118
[...]

Canceling the timer before starting it ensure that the timer callback is
not running when the timer is started, solving this race condition.

Fixes: 0cc67945ea59 ("mailbox: switch to hrtimer for tx_complete polling")
Reported-by: Da Xue <da at libre.computer>
Signed-off-by: Jerome Brunet <jbrunet at baylibre.com>
---
 drivers/mailbox/mailbox.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 0b821a5b2db8..34f9ab01caef 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -82,9 +82,13 @@ static void msg_submit(struct mbox_chan *chan)
 exit:
 	spin_unlock_irqrestore(&chan->lock, flags);
 
-	if (!err && (chan->txdone_method & TXDONE_BY_POLL))
-		/* kick start the timer immediately to avoid delays */
+	if (!err && (chan->txdone_method & TXDONE_BY_POLL)) {
+		/* Disable the timer if already active ... */
+		hrtimer_cancel(&chan->mbox->poll_hrt);
+
+		/* ... and kick start it immediately to avoid delays */
 		hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
+	}
 }
 
 static void tx_tick(struct mbox_chan *chan, int r)
-- 
2.25.4