linux-kernel - Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias

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Message-ID: <20190902164604.1d04614f.cohuck@redhat.com>
Date:   Mon, 2 Sep 2019 16:46:04 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias

On Fri, 30 Aug 2019 15:45:13 +0000
Parav Pandit <parav@...lanox.com> 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.

> 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.