Re: [PATCH 02/12] Bluetooth: Add basic start/complete HCI transaction functions

[Johan Hedberg <johan.hedberg@gmail.com>]

Hi Szymon,

On Fri, Feb 15, 2013, Szymon Janc wrote:
> > > > > > +int hci_start_transaction(struct hci_dev *hdev)
> > > > > > +{
> > > > > > +       struct hci_transaction *transaction;
> > > > > > +       int err;
> > > > > > +
> > > > > > +       hci_transaction_lock(hdev);
> > > > > > +
> > > > > > +       /* We can't start a new transaction if there's another one in
> > > > > > +        * progress of being built.
> > > > > > +        */
> > > > > > +       if (hdev->build_transaction) {
> > > > > > +               err = -EBUSY;
> > > > > > +               goto unlock;
> > > > > > +       }
> > > > > > +
> > > > > > +       transaction = kmalloc(sizeof(*transaction), GFP_ATOMIC);
> > > > > 
> > > > > I've failed to see why we need GFP_ATOMIC here. As this code is not
> > > > > running in any atomic section, we can allocate memory using
> > > > > GFP_KERNEL.
> > > > 
> > > > Since one of the intentions of this API is to create an async version of
> > > > hci_request() I think it's better to keep GFP_ATOMIC here. One situation
> > > > where you couldn't for sure use hci_request() is if you're in an atomic
> > > > section and then a HCI request would be the only other alternative.
> > > 
> > > You lock mutex in hci_start_transaction so it is non-atomic anyway..
> > 
> > This has me a bit confused since hci_dev_lock() also uses the same. Does
> > this mean that there's no code at all in the Bluetooth subsystem anymore
> > that can run in atomic context? (since most things require locking at
> > least the hdev struct). I remember seeing some time back (after the
> > whole workqueue conversion) bugs arising from incorrect GFP_ATOMIC ->
> > GFP_KERNEL conversions (none of which luckily made it to Linus' tree), so at
> > least some parts seemed to still be atomic. Do a grep for GFP_ATOMIC in
> > net/bluetooth/ and you'll find plenty of them.
> 
> The bug (by me btw:P) you are probably referring to was due to rwlock which
> is spin lock and raise sleeping in atomic warning even if run e.g. from wq.

Alright. I've just sent a v3 of the set which uses GFP_KERNEL for all
allocations.

Johan