linux-kernel - [PATCH] mailbox: forward the hrtimer if not queued and under a lock

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Message-ID: <20220331070115.29421-1-bjorn.ardo@axis.com>
Date:   Thu, 31 Mar 2022 09:01:15 +0200
From:   Björn Ardö <bjorn.ardo@...s.com>
To:     Jassi Brar <jassisinghbrar@...il.com>
CC:     <kernel@...s.com>,
        Björn Ardö <bjorn.ardo@...s.com>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH] mailbox: forward the hrtimer if not queued and under a lock

This reverts commit c7dacf5b0f32957b24ef29df1207dc2cd8307743,
"mailbox: avoid timer start from callback"

The previous commit was reverted since it lead to a race that
caused the hrtimer to not be started at all. The check for
hrtimer_active() in msg_submit() will return true if the
callback function txdone_hrtimer() is currently running. This
function could return HRTIMER_NORESTART and then the timer
will not be restarted, and also msg_submit() will not start
the timer. This will lead to a message actually being submitted
but no timer will start to check for its compleation.

The original fix that added checking hrtimer_active() was added to
avoid a warning with hrtimer_forward. Looking in the kernel
another solution to avoid this warning is to check hrtimer_is_queued()
before calling hrtimer_forward_now() instead. This however requires a
lock so the timer is not started by msg_submit() inbetween this check
and the hrtimer_forward() call.

Signed-off-by: Björn Ardö <bjorn.ardo@...s.com>
---
 drivers/mailbox/mailbox.c          | 19 +++++++++++++------
 include/linux/mailbox_controller.h |  1 +
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 3e7d4b20ab34..4229b9b5da98 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -82,11 +82,11 @@ static void msg_submit(struct mbox_chan *chan)
 exit:
 	spin_unlock_irqrestore(&chan->lock, flags);
 
-	/* kick start the timer immediately to avoid delays */
 	if (!err && (chan->txdone_method & TXDONE_BY_POLL)) {
-		/* but only if not already active */
-		if (!hrtimer_active(&chan->mbox->poll_hrt))
-			hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
+		/* kick start the timer immediately to avoid delays */
+		spin_lock_irqsave(&chan->mbox->poll_hrt_lock, flags);
+		hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
+		spin_unlock_irqrestore(&chan->mbox->poll_hrt_lock, flags);
 	}
 }
 
@@ -120,20 +120,26 @@ static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer)
 		container_of(hrtimer, struct mbox_controller, poll_hrt);
 	bool txdone, resched = false;
 	int i;
+	unsigned long flags;
 
 	for (i = 0; i < mbox->num_chans; i++) {
 		struct mbox_chan *chan = &mbox->chans[i];
 
 		if (chan->active_req && chan->cl) {
-			resched = true;
 			txdone = chan->mbox->ops->last_tx_done(chan);
 			if (txdone)
 				tx_tick(chan, 0);
+			else
+				resched = true;
 		}
 	}
 
 	if (resched) {
-		hrtimer_forward_now(hrtimer, ms_to_ktime(mbox->txpoll_period));
+		spin_lock_irqsave(&mbox->poll_hrt_lock, flags);
+		if (!hrtimer_is_queued(hrtimer))
+			hrtimer_forward_now(hrtimer, ms_to_ktime(mbox->txpoll_period));
+		spin_unlock_irqrestore(&mbox->poll_hrt_lock, flags);
+
 		return HRTIMER_RESTART;
 	}
 	return HRTIMER_NORESTART;
@@ -500,6 +506,7 @@ int mbox_controller_register(struct mbox_controller *mbox)
 		hrtimer_init(&mbox->poll_hrt, CLOCK_MONOTONIC,
 			     HRTIMER_MODE_REL);
 		mbox->poll_hrt.function = txdone_hrtimer;
+		spin_lock_init(&mbox->poll_hrt_lock);
 	}
 
 	for (i = 0; i < mbox->num_chans; i++) {
diff --git a/include/linux/mailbox_controller.h b/include/linux/mailbox_controller.h
index 36d6ce673503..6fee33cb52f5 100644
--- a/include/linux/mailbox_controller.h
+++ b/include/linux/mailbox_controller.h
@@ -83,6 +83,7 @@ struct mbox_controller {
 				      const struct of_phandle_args *sp);
 	/* Internal to API */
 	struct hrtimer poll_hrt;
+	spinlock_t poll_hrt_lock;
 	struct list_head node;
 };
 
-- 
2.20.1