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

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Date: Fri, 5 Jan 2024 11:05:21 -0500
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Jonas Dreßler <verdre@...d.nl>
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
Subject: Re: [PATCH 3/5] Bluetooth: hci_event: Remove limit of 2 reconnection attempts

Hi Jonas,

On Fri, Jan 5, 2024 at 10:54 AM Jonas Dreßler <verdre@...d.nl> wrote:
>
> 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.

Perhaps I'm missing something, but it should be possible to block
concurrent access to HCI while a command is pending with use of
hci_cmd_sync, at least on LE we do that by waiting the connection
complete event so connection attempts are serialized but we don't seem
to be doing the same for BR/EDR.


> >
> >> 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



--
Luiz Augusto von Dentz