[PATCH RFC] mailbox: fix hrtimer_forward() warning

[Alexey Klimov]

When mailbox controller provides two or more channels,
timer-based polling is used by mailbox controller
and channels are actively used by mailbox client(s) it's very
easy to trigger the warning in hrtimer_forward():

[  247.853060] WARNING: CPU: 6 PID: 0 at kernel/time/hrtimer.c:805 hrtimer_forward+0x88/0xd8
[  247.853549] Modules linked in:
[  247.853907] CPU: 6 PID: 0 Comm: swapper/6 Tainted: G        W       4.11.0-rc2-00362-g93afaa4513bb-dirty #13
[  247.854472] Hardware name: linux,dummy-virt (DT)
[  247.854699] task: ffff80001d89d780 task.stack: ffff80001d8c4000
[  247.854999] PC is at hrtimer_forward+0x88/0xd8
[  247.855280] LR is at txdone_hrtimer+0xd4/0xf8
[  247.855551] pc : [<ffff0000081039f0>] lr : [<ffff00000881b874>] pstate: 200001c5
[  247.855857] sp : ffff80001efbdeb0
[  247.856072] x29: ffff80001efbdeb0 x28: ffff80001efc3140
[  247.856358] x27: ffff00000881b7a0 x26: 00000039ac93e8b6
[  247.856604] x25: ffff000008e756be x24: ffff80001c4a1348
[  247.856882] x23: 0000000000000001 x22: 00000000000000f8
[  247.857189] x21: ffff80001c4a1318 x20: ffff80001d327110
[  247.857509] x19: 00000000000f4240 x18: 0000000000000030
[  247.857808] x17: 0000ffffaecdf370 x16: ffff0000081ccc80
[  247.858000] x15: 0000000000000010 x14: 00000000fffffff0
[  247.858186] x13: ffff000008f488e0 x12: 000000000002e3eb
[  247.858381] x11: ffff000008979690 x10: 0000000000000000
[  247.858573] x9 : 0000000000000001 x8 : ffff80001efc66e0
[  247.858758] x7 : ffff80001efc6708 x6 : 00000005be7732f2
[  247.858943] x5 : 0000000000000001 x4 : ffff80001c4a1348
[  247.859130] x3 : 00000039ac94952a x2 : 00000000000f4240
[  247.859315] x1 : 00000039ac98243c x0 : 0000000000038f12
[  247.859582] ---[ end trace d61812426ec3c30b ]---

To fix it this patch introduces one more mbox spinlock for hr-timer and
hrt_active variable in controller structure.
The hr-timer callback restarts timer unless there no active requests
on any channel.

Signed-off-by: Alexey Klimov <alexey.klimov@xxxxxxx>
---
 drivers/mailbox/mailbox.c          | 51 ++++++++++++++++++++++++++++++--------
 include/linux/mailbox_controller.h |  2 ++
 2 files changed, 42 insertions(+), 11 deletions(-)

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 9dfbf7e..57ae422 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -85,9 +85,17 @@ 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 */
-		hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
+	if (!err && (chan->txdone_method & TXDONE_BY_POLL)) {
+
+		spin_lock_irqsave(&chan->mbox->lock_hrt, flags);
+		/* try to start the timer immediately to avoid delays */
+		if (!chan->mbox->hrt_active) {
+			chan->mbox->hrt_active = 1;
+			hrtimer_start(&chan->mbox->poll_hrt, 0,
+							HRTIMER_MODE_REL);
+		}
+		spin_unlock_irqrestore(&chan->mbox->lock_hrt, flags);
+	}
 }
 
 static void tx_tick(struct mbox_chan *chan, int r)
@@ -121,23 +129,43 @@ static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer)
 	bool txdone, resched = false;
 	int i;
 
+	spin_lock(&mbox->lock_hrt);
+
 	for (i = 0; i < mbox->num_chans; i++) {
 		struct mbox_chan *chan = &mbox->chans[i];
 
+		/*
+		 * Reading of active_req is racy and we may miss an active
+		 * request. However, the enqueuing thread will spin on the
+		 * HR-timer lock, which we acquired here, and will start the
+		 * timer if required.
+		 */
 		if (chan->active_req && chan->cl) {
-			txdone = chan->mbox->ops->last_tx_done(chan);
-			if (txdone)
+			/*
+			 * If we observe an active request on any channel, then
+			 * we have to re-start the timer; if we see that
+			 * a request has finished, we don't know if tx_tick()
+			 * will schedule the next request or not. Therefore we
+			 * have to re-sched the HR-timer here in all cases.
+			 */
+			resched = true;
+
+			txdone = mbox->ops->last_tx_done(chan);
+			if (txdone) {
+				spin_unlock(&mbox->lock_hrt);
 				tx_tick(chan, 0);
-			else
-				resched = true;
+				spin_lock(&mbox->lock_hrt);
+			}
 		}
 	}
 
-	if (resched) {
+	if (resched)
 		hrtimer_forward_now(hrtimer, ms_to_ktime(mbox->txpoll_period));
-		return HRTIMER_RESTART;
-	}
-	return HRTIMER_NORESTART;
+	else
+		mbox->hrt_active = 0;
+	spin_unlock(&mbox->lock_hrt);
+
+	return resched ? HRTIMER_RESTART : HRTIMER_NORESTART;
 }
 
 /**
@@ -462,6 +490,7 @@ int mbox_controller_register(struct mbox_controller *mbox)
 			return -EINVAL;
 		}
 
+		spin_lock_init(&mbox->lock_hrt);
 		hrtimer_init(&mbox->poll_hrt, CLOCK_MONOTONIC,
 			     HRTIMER_MODE_REL);
 		mbox->poll_hrt.function = txdone_hrtimer;
diff --git a/include/linux/mailbox_controller.h b/include/linux/mailbox_controller.h
index 74deadb..1dd4c47 100644
--- a/include/linux/mailbox_controller.h
+++ b/include/linux/mailbox_controller.h
@@ -83,6 +83,8 @@ struct mbox_controller {
 				      const struct of_phandle_args *sp);
 	/* Internal to API */
 	struct hrtimer poll_hrt;
+	spinlock_t lock_hrt;
+	unsigned int hrt_active;
 	struct list_head node;
 };
 
-- 
1.9.1