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Date:   Sun, 15 Oct 2017 03:40:45 -0700
From:   =?UTF-8?q?Ronald=20Tschal=C3=A4r?= 
        <ronald@...ovation.ch>
To:     Marcel Holtmann <marcel@...tmann.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Johan Hedberg <johan.hedberg@...il.com>
Cc:     Lukas Wunner <lukas@...ner.de>,
        Dean Jenkins <Dean_Jenkins@...tor.com>,
        linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] Bluetooth: hci_ldisc: Allow sleeping while proto locks are
 held.

Commit dec2c92880cc5435381d50e3045ef018a762a917 ("Bluetooth: hci_ldisc:
Use rwlocking to avoid closing proto races") introduced locks in
hci_ldisc that are held while calling the proto functions. These locks
are rwlock's, and hence do not allow sleeping while they are held.
However, the proto functions that hci_bcm registers use mutexes and
hence need to be able to sleep.

In more detail: hci_uart_tty_receive() and hci_uart_dequeue() both
acquire the rwlock, after which they call proto->recv() and
proto->dequeue(), respectively. In the case of hci_bcm these point to
bcm_recv() and bcm_dequeue(). The latter both acquire the
bcm_device_lock, which is a mutex, so doing so results in a call to
might_sleep(). But since we're holding a rwlock in hci_ldisc, that
results in the following BUG (this for the dequeue case - a similar
one for the receive case is omitted for brevity):

  BUG: sleeping function called from invalid context at kernel/locking/mutex.c
  in_atomic(): 1, irqs_disabled(): 0, pid: 7303, name: kworker/7:3
  INFO: lockdep is turned off.
  CPU: 7 PID: 7303 Comm: kworker/7:3 Tainted: G        W  OE   4.13.2+ #17
  Hardware name: Apple Inc. MacBookPro13,3/Mac-A5C67F76ED83108C, BIOS MBP133.8
  Workqueue: events hci_uart_write_work [hci_uart]
  Call Trace:
   dump_stack+0x8e/0xd6
   ___might_sleep+0x164/0x250
   __might_sleep+0x4a/0x80
   __mutex_lock+0x59/0xa00
   ? lock_acquire+0xa3/0x1f0
   ? lock_acquire+0xa3/0x1f0
   ? hci_uart_write_work+0xd3/0x160 [hci_uart]
   mutex_lock_nested+0x1b/0x20
   ? mutex_lock_nested+0x1b/0x20
   bcm_dequeue+0x21/0xc0 [hci_uart]
   hci_uart_write_work+0xe6/0x160 [hci_uart]
   process_one_work+0x253/0x6a0
   worker_thread+0x4d/0x3b0
   kthread+0x133/0x150

We can't replace the mutex in hci_bcm, because there are other calls
there that might sleep. Therefore this replaces the rwlock's in
hci_ldisc with rw_semaphore's (which allow sleeping). This is a safer
approach anyway as it reduces the restrictions on the proto callbacks.
Also, because acquiring write-lock is very rare compared to acquiring
the read-lock, the percpu variant of rw_semaphore is used.

Lastly, the read-lock in the hci_uart_tx_wakeup() callback now needed
to be removed because that function is called from an IRQ context. But
since it doesn't actually call any proto functions, instead just
queueing the work, and the other operations are atomic bit operations,
this is ok.

Signed-off-by: Ronald Tschalär <ronald@...ovation.ch>
Cc: Lukas Wunner <lukas@...ner.de>
Cc: Marcel Holtmann <marcel@...tmann.org>
Cc: Gustavo Padovan <gustavo@...ovan.org>
Cc: Johan Hedberg <johan.hedberg@...il.com>
Cc: Dean Jenkins <Dean_Jenkins@...tor.com>
---
 drivers/bluetooth/hci_ldisc.c | 31 +++++++++++++------------------
 drivers/bluetooth/hci_uart.h  |  2 +-
 2 files changed, 14 insertions(+), 19 deletions(-)

diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index eec95019f15c..08bcb1239aa0 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -115,12 +115,12 @@ static inline struct sk_buff *hci_uart_dequeue(struct hci_uart *hu)
 	struct sk_buff *skb = hu->tx_skb;
 
 	if (!skb) {
-		read_lock(&hu->proto_lock);
+		percpu_down_read(&hu->proto_lock);
 
 		if (test_bit(HCI_UART_PROTO_READY, &hu->flags))
 			skb = hu->proto->dequeue(hu);
 
-		read_unlock(&hu->proto_lock);
+		percpu_up_read(&hu->proto_lock);
 	} else {
 		hu->tx_skb = NULL;
 	}
@@ -130,8 +130,6 @@ static inline struct sk_buff *hci_uart_dequeue(struct hci_uart *hu)
 
 int hci_uart_tx_wakeup(struct hci_uart *hu)
 {
-	read_lock(&hu->proto_lock);
-
 	if (!test_bit(HCI_UART_PROTO_READY, &hu->flags))
 		goto no_schedule;
 
@@ -145,8 +143,6 @@ int hci_uart_tx_wakeup(struct hci_uart *hu)
 	schedule_work(&hu->write_work);
 
 no_schedule:
-	read_unlock(&hu->proto_lock);
-
 	return 0;
 }
 EXPORT_SYMBOL_GPL(hci_uart_tx_wakeup);
@@ -247,12 +243,12 @@ static int hci_uart_flush(struct hci_dev *hdev)
 	tty_ldisc_flush(tty);
 	tty_driver_flush_buffer(tty);
 
-	read_lock(&hu->proto_lock);
+	percpu_down_read(&hu->proto_lock);
 
 	if (test_bit(HCI_UART_PROTO_READY, &hu->flags))
 		hu->proto->flush(hu);
 
-	read_unlock(&hu->proto_lock);
+	percpu_up_read(&hu->proto_lock);
 
 	return 0;
 }
@@ -275,15 +271,15 @@ static int hci_uart_send_frame(struct hci_dev *hdev, struct sk_buff *skb)
 	BT_DBG("%s: type %d len %d", hdev->name, hci_skb_pkt_type(skb),
 	       skb->len);
 
-	read_lock(&hu->proto_lock);
+	percpu_down_read(&hu->proto_lock);
 
 	if (!test_bit(HCI_UART_PROTO_READY, &hu->flags)) {
-		read_unlock(&hu->proto_lock);
+		percpu_up_read(&hu->proto_lock);
 		return -EUNATCH;
 	}
 
 	hu->proto->enqueue(hu, skb);
-	read_unlock(&hu->proto_lock);
+	percpu_up_read(&hu->proto_lock);
 
 	hci_uart_tx_wakeup(hu);
 
@@ -486,7 +482,7 @@ static int hci_uart_tty_open(struct tty_struct *tty)
 	INIT_WORK(&hu->init_ready, hci_uart_init_work);
 	INIT_WORK(&hu->write_work, hci_uart_write_work);
 
-	rwlock_init(&hu->proto_lock);
+	percpu_init_rwsem(&hu->proto_lock);
 
 	/* Flush any pending characters in the driver */
 	tty_driver_flush_buffer(tty);
@@ -503,7 +499,6 @@ static void hci_uart_tty_close(struct tty_struct *tty)
 {
 	struct hci_uart *hu = tty->disc_data;
 	struct hci_dev *hdev;
-	unsigned long flags;
 
 	BT_DBG("tty %p", tty);
 
@@ -520,9 +515,9 @@ static void hci_uart_tty_close(struct tty_struct *tty)
 	cancel_work_sync(&hu->write_work);
 
 	if (test_bit(HCI_UART_PROTO_READY, &hu->flags)) {
-		write_lock_irqsave(&hu->proto_lock, flags);
+		percpu_down_write(&hu->proto_lock);
 		clear_bit(HCI_UART_PROTO_READY, &hu->flags);
-		write_unlock_irqrestore(&hu->proto_lock, flags);
+		percpu_up_write(&hu->proto_lock);
 
 		if (hdev) {
 			if (test_bit(HCI_UART_REGISTERED, &hu->flags))
@@ -582,10 +577,10 @@ static void hci_uart_tty_receive(struct tty_struct *tty, const u8 *data,
 	if (!hu || tty != hu->tty)
 		return;
 
-	read_lock(&hu->proto_lock);
+	percpu_down_read(&hu->proto_lock);
 
 	if (!test_bit(HCI_UART_PROTO_READY, &hu->flags)) {
-		read_unlock(&hu->proto_lock);
+		percpu_up_read(&hu->proto_lock);
 		return;
 	}
 
@@ -593,7 +588,7 @@ static void hci_uart_tty_receive(struct tty_struct *tty, const u8 *data,
 	 * tty caller
 	 */
 	hu->proto->recv(hu, data, count);
-	read_unlock(&hu->proto_lock);
+	percpu_up_read(&hu->proto_lock);
 
 	if (hu->hdev)
 		hu->hdev->stat.byte_rx += count;
diff --git a/drivers/bluetooth/hci_uart.h b/drivers/bluetooth/hci_uart.h
index d9cd95d81149..66e8c68e4607 100644
--- a/drivers/bluetooth/hci_uart.h
+++ b/drivers/bluetooth/hci_uart.h
@@ -87,7 +87,7 @@ struct hci_uart {
 	struct work_struct	write_work;
 
 	const struct hci_uart_proto *proto;
-	rwlock_t		proto_lock;	/* Stop work for proto close */
+	struct percpu_rw_semaphore proto_lock;	/* Stop work for proto close */
 	void			*priv;
 
 	struct sk_buff		*tx_skb;
-- 
2.13.6

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