> -----Original Message----- > From: Cornelia Huck <cohuck@xxxxxxxxxx> > Sent: Monday, September 2, 2019 8:16 PM > To: Parav Pandit <parav@xxxxxxxxxxxx> > Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>; > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx > Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias > > On Fri, 30 Aug 2019 15:45:13 +0000 > Parav Pandit <parav@xxxxxxxxxxxx> wrote: > > > > > > > > This detour via the local variable looks weird to me. Can > > > > > > > you either create the alias directly in the mdev (would need > > > > > > > to happen later in the function, but I'm not sure why you > > > > > > > generate the alias before checking for duplicates anyway), or do > an explicit copy? > > > > > > Alias duplicate check is done after generating it, because > > > > > > duplicate alias are > > > > > not allowed. > > > > > > The probability of collision is rare. > > > > > > So it is speculatively generated without hold the lock, > > > > > > because there is no > > > > > need to hold the lock. > > > > > > It is compared along with guid while mutex lock is held in single > loop. > > > > > > And if it is duplicate, there is no need to allocate mdev. > > > > > > > > > > > > It will be sub optimal to run through the mdev list 2nd time > > > > > > after mdev > > > > > creation and after generating alias for duplicate check. > > > > > > > > > > Ok, but what about copying it? I find this "set local variable > > > > > to NULL after ownership is transferred" pattern a bit unintuitive. > > > > > Copying it to the mdev (and then unconditionally freeing it) > > > > > looks more > > > obvious to me. > > > > Its not unconditionally freed. > > > > > > That's not what I have been saying :( > > > > > Ah I see. You want to allocate alias memory twice; once inside mdev device > and another one in _create() function. > > _create() one you want to free unconditionally. > > > > Well, passing pointer is fine. > > It's not that it doesn't work, but it feels fragile due to its non-obviousness. And its well commented as Alex asked. > > > mdev_register_device() has similar little tricky pattern that makes parent = > NULL on __find_parent_device() finds duplicate one. > > I don't think that the two are comparable. > They are very similar. Why parent should be marked null otherwise. > > > > Ownership transfer is more straight forward code. > > I have to disagree here. > Ok. It is better than allocating memory twice. So I prefer to stick to this method. > > > > It is similar to device_initialize(), device init sequence code, where once > device_initialize is done, freeing the device memory will be left to the > put_device(), we don't call kfree() on mdev device. > > This does not really look similar to me: devices are refcounted structures, > while strings aren't; you transfer a local pointer to a refcounted structure > and then discard the local reference.
