Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias
From: Cornelia Huck
Date: Mon Sep 02 2019 - 10:46:12 EST
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
> 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.
> Ownership transfer is more straight forward code.
I have to disagree here.
> 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.