Re: [PATCH] Bluetooth: Use non locking get_chan and explicit lock

[Emeltchenko Andrei <Andrei.Emeltchenko.news@gmail.com>]

Hi Marcel,

On Sat, Jan 21, 2012 at 05:41:34PM +0100, Marcel Holtmann wrote:
> Hi Andrei,
> 
> > For receiving ACL packets use __l2cap_get_chan_by_scid which is not
> > locking sk and explicitly lock sk after checking that it is exist.
> > Code looks nicer since now it is surrounded by lock/release.
> > 
> > Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
> > ---
> >  net/bluetooth/l2cap_core.c |    7 +++++--
> >  1 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > index d16ad49..53dbfd3 100644
> > --- a/net/bluetooth/l2cap_core.c
> > +++ b/net/bluetooth/l2cap_core.c
> > @@ -4243,7 +4243,7 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
> >  	u16 tx_seq;
> >  	int len;
> >  
> > -	chan = l2cap_get_chan_by_scid(conn, cid);
> > +	chan = __l2cap_get_chan_by_scid(conn, cid);
> >  	if (!chan) {
> >  		if (cid == L2CAP_CID_A2MP) {
> >  			chan = a2mp_channel_create(conn, skb);
> > @@ -4255,6 +4255,8 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
> >  	}
> >  
> >  	sk = chan->sk;
> > +	if (sk)
> > +		lock_sock(sk);
> 
> this is the part I clearly do not like. This is pretty nasty conditional
> locking. We need to figure out something better.

Actually l2cap_data_channel function ends with:

        <------8<--------------------
        |  if (sk)
        |          release_sock(sk);
        |
        <------8<--------------------

So in _this_ case it looks consistent. But I agree that generally it does
not looks nice. The reason for that is l2cap_chan is locked with socket
lock. Maybe we could create mutex l2cap_lock and functions like
lock_chan(chan) / unlock_chan(chan) and change socket locks in l2cap_core
with these new locks?

Best regards 
Andrei Emeltchenko