linux-kernel - Re: [PATCH 3/5] Bluetooth: hci_event: Remove limit of 2 reconnection attempts

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Date: Fri, 5 Jan 2024 16:54:47 +0100
From: Jonas Dreßler <verdre@...d.nl>
To: Luiz Augusto von Dentz <luiz.dentz@...il.com>
Cc: Marcel Holtmann <marcel@...tmann.org>,
 Johan Hedberg <johan.hedberg@...il.com>, linux-bluetooth@...r.kernel.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org, verdre@...d.nl
Subject: Re: [PATCH 3/5] Bluetooth: hci_event: Remove limit of 2 reconnection
 attempts

Hi Luiz,

On 1/3/24 17:05, Luiz Augusto von Dentz wrote:
> Hi Jonas,
> 
> On Tue, Jan 2, 2024 at 1:59 PM Jonas Dreßler <verdre@...d.nl> wrote:
>>
>> Since commit 4c67bc74f016b0d360b8573e18969c0ff7926974, we retry connecting
>> later when we get a "Command Disallowed" error returned by "Create
>> Connection".
>>
>> In this commit the intention was to retry only once, and give up if we see
>> "Command Disallowed" again on the second try.
>>
>> This made sense back then when the retry was initiated *only* from the
>> "Connect Complete" event. If we received that event, we knew that now the
>> card now must have a "free slot" for a new connection request again. These
>> days we call hci_conn_check_pending() from a few more places though, and
>> in these places we can't really be sure that there's a "free slot" on the
>> card, so the second try to "Create Connection" might fail again.
>>
>> Deal with this by being less strict about these retries and try again
>> every time we get "Command Disallowed" errors, removing the limitation to
>> only two attempts.
>>
>> Since this can potentially cause us to enter an endless cycle of
>> reconnection attempts, we'll add some guarding against that with the next
>> commit.
> 
> Don't see where you are doing such guarding, besides you seem to
> assume HCI_ERROR_COMMAND_DISALLOWED would always means the controller
> is busy, or something like that, but it could perform the connection
> later, but that may not always be the case, thus why I think
> reconnecting just a few number of times is better, if you really need
> to keep retrying then this needs to be controlled by a policy in
> userspace not hardcoded in the kernel, well I can even argument that
> perhaps the initial number of reconnection shall be configurable so
> one don't have to recompile the kernel if that needs changing.

Yes, fair enough, the next commit assumes that COMMAND_DISALLOWED always 
means busy. The guarding is that we stop retrying as soon as there's no 
(competing) ongoing connection attempt nor an active inquiry, which 
should eventually be the case no matter what, no?

I agree it's probably still better to not rely on this fairly complex 
sanity check and keep the checking of attempts nonetheless.

I think we could keep doing that if we check for 
!hci_conn_hash_lookup_state(hdev, ACL_LINK, BT_CONNECT) && 
!test_bit(HCI_INQUIRY, &hdev->flags) in hci_conn_check_pending() before 
we actually retry, to make sure the retry counter doesn't get 
incremented wrongly. I'll give that a try.

> 
>> Signed-off-by: Jonas Dreßler <verdre@...d.nl>
>> ---
>>   net/bluetooth/hci_event.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/bluetooth/hci_event.c b/net/bluetooth/hci_event.c
>> index e8b4a0126..e1f5b6f90 100644
>> --- a/net/bluetooth/hci_event.c
>> +++ b/net/bluetooth/hci_event.c
>> @@ -2323,12 +2323,13 @@ static void hci_cs_create_conn(struct hci_dev *hdev, __u8 status)
>>
>>          if (status) {
>>                  if (conn && conn->state == BT_CONNECT) {
>> -                       if (status != HCI_ERROR_COMMAND_DISALLOWED || conn->attempt > 2) {
>> +                       if (status == HCI_ERROR_COMMAND_DISALLOWED) {
>> +                               conn->state = BT_CONNECT2;
>> +                       } else {
>>                                  conn->state = BT_CLOSED;
>>                                  hci_connect_cfm(conn, status);
>>                                  hci_conn_del(conn);
>> -                       } else
>> -                               conn->state = BT_CONNECT2;
>> +                       }
>>                  }
>>          } else {
>>                  if (!conn) {
>> --
>> 2.43.0
>>
> 
> 

Cheers,
Jonas