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From:   Duoming Zhou <duoming@zju.edu.cn>
To:     linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org
Cc:     peter.chen@kernel.org, gregkh@linuxfoundation.org,
        Duoming Zhou <duoming@zju.edu.cn>
Subject: [PATCH] usb: chipidea: fix deadlock in ci_otg_del_timer
Date:   Sun, 18 Sep 2022 11:33:12 +0800
Message-Id: <20220918033312.94348-1-duoming@zju.edu.cn>
Precedence: bulk

There is a deadlock in ci_otg_del_timer(), the process is
shown below:

    (thread 1)                  |        (thread 2)
ci_otg_del_timer()              | ci_otg_hrtimer_func()
  ...                           |
  spin_lock_irqsave() //(1)     |  ...
  ...                           |
  hrtimer_cancel()              |  spin_lock_irqsave() //(2)
  (block forever)

We hold ci->lock in position (1) and use hrtimer_cancel() to
wait ci_otg_hrtimer_func() to stop, but ci_otg_hrtimer_func()
also need ci->lock in position (2). As a result, the
hrtimer_cancel() in ci_otg_del_timer() will be blocked forever.

This patch extracts hrtimer_cancel() from the protection of
spin_lock_irqsave() in order that the ci_otg_hrtimer_func()
could obtain the ci->lock.

What`s more, there will be no race happen. Because the
"next_timer" is always under the protection of
spin_lock_irqsave() and we only check whether "next_timer"
equals to NUM_OTG_FSM_TIMERS in the following code.

Fixes: 3a316ec4c91c ("usb: chipidea: use hrtimer for otg fsm timers")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
 drivers/usb/chipidea/otg_fsm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/chipidea/otg_fsm.c b/drivers/usb/chipidea/otg_fsm.c
index 61b157b9c66..a7858462428 100644
--- a/drivers/usb/chipidea/otg_fsm.c
+++ b/drivers/usb/chipidea/otg_fsm.c
@@ -256,8 +256,10 @@ static void ci_otg_del_timer(struct ci_hdrc *ci, enum otg_fsm_timer t)
 	ci->enabled_otg_timer_bits &= ~(1 << t);
 	if (ci->next_otg_timer == t) {
 		if (ci->enabled_otg_timer_bits == 0) {
+			spin_unlock_irqrestore(&ci->lock, flags);
 			/* No enabled timers after delete it */
 			hrtimer_cancel(&ci->otg_fsm_hrtimer);
+			spin_lock_irqsave(&ci->lock, flags);
 			ci->next_otg_timer = NUM_OTG_FSM_TIMERS;
 		} else {
 			/* Find the next timer */
-- 
2.17.1