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Subject[PATCH v2] 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, because hci_uart_tx_wakeup() may be called from an IRQ context,
we can't block (sleep) while trying acquire the read lock there, so we
use the trylock variant.

Signed-off-by: Ronald Tschalär <ronald@innovation.ch>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: Dean Jenkins <Dean_Jenkins@mentor.com>
---
Changes in v2:
- Add back locking in hci_uart_tx_wakeup(). Removing the locking
altogether there was wrong, as nicely pointed out by Dean Jenkins.

drivers/bluetooth/hci_ldisc.c | 38 ++++++++++++++++++++++----------------
drivers/bluetooth/hci_uart.h | 2 +-
2 files changed, 23 insertions(+), 17 deletions(-)

diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index eec95019f15c..31def781a562 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;
}
@@ -130,7 +130,14 @@ 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);
+ /* This may be called in an IRQ context, so we can't sleep. Therefore
+ * we try to acquire the lock only, and if that fails we assume the
+ * tty is being closed because that is the only time the write lock is
+ * acquired. If, however, at some point in the future the write lock
+ * is also acquired in other situations, then this must be revisited.
+ */
+ if (!percpu_down_read_trylock(&hu->proto_lock))
+ return 0;

if (!test_bit(HCI_UART_PROTO_READY, &hu->flags))
goto no_schedule;
@@ -145,7 +152,7 @@ int hci_uart_tx_wakeup(struct hci_uart *hu)
schedule_work(&hu->write_work);

no_schedule:
- read_unlock(&hu->proto_lock);
+ percpu_up_read(&hu->proto_lock);

return 0;
}
@@ -247,12 +254,12 @@ 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 +282,15 @@ 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 +493,7 @@ 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 +510,6 @@ 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 +526,9 @@ 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 +588,10 @@ 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 +599,7 @@ 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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