[PATCH 24/24] Bluetooth: Fix write_room() calculation

Peter Hurley
2014-02-10T02:20:32Z

The skb truesize of a 12-byte payload with a 10-byte head/tail
reserve is 768 bytes. Consequently, even with 40 tx_credits, at
most 6 packets could be queued at any one time:

40 tx_credits * 127-byte mtu < 768-byte truesize * 7

This error could also cause the tx queue to apparently stall if
credit flow control is disabled (where tx_credits is fixed at 5),
or if the receiver only granted a limited number of tx credits
(eg., less than 7).

Instead, track the outstanding number of queued packets not yet sent
in wmem_alloc and allow for a maximum of 40 queued packets. Report
the space avail for a single write() as the mtu * number of packets
left before reaching the maximum.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
net/bluetooth/rfcomm/tty.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index 3f44195..403ec09 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -352,20 +352,16 @@ static inline unsigned int rfcomm_room(struct rfcomm_dev *dev)
{
struct rfcomm_dlc *dlc = dev->dlc;

- /* The limit is bogus; the number of packets which can
- * currently be sent by the krfcommd thread has no relevance
- * to the number of packets which can be queued on the dlc's
- * tx queue.
- */
- int limit = dlc->mtu * (dlc->tx_credits?:1);
+ /* Limit the outstanding number of packets not yet sent to 40 */
+ int pending = 40 - atomic_read(&dev->wmem_alloc);

- return max(0, limit - atomic_read(&dev->wmem_alloc));
+ return max(0, pending) * dlc->mtu;
}

static void rfcomm_wfree(struct sk_buff *skb)
{
struct rfcomm_dev *dev = (void *) skb->sk;
- atomic_sub(skb->truesize, &dev->wmem_alloc);
+ atomic_dec(&dev->wmem_alloc);
if (test_bit(RFCOMM_TTY_ATTACHED, &dev->flags))
tty_port_tty_wakeup(&dev->port);
tty_port_put(&dev->port);
@@ -374,7 +370,7 @@ static void rfcomm_wfree(struct sk_buff *skb)
static void rfcomm_set_owner_w(struct sk_buff *skb, struct rfcomm_dev *dev)
{
tty_port_get(&dev->port);
- atomic_add(skb->truesize, &dev->wmem_alloc);
+ atomic_inc(&dev->wmem_alloc);
skb->sk = (void *) dev;
skb->destructor = rfcomm_wfree;
}
--
1.8.1.2