linux-kernel - Re: [PATCH v3 46/57] perf: Simplify pmu_dev

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Message-ID: <2023061213-knapsack-moonlike-e595@gregkh>
Date:   Mon, 12 Jun 2023 14:18:03 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH v3 46/57] perf: Simplify pmu_dev_alloc()

On Mon, Jun 12, 2023 at 11:44:00AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 12, 2023 at 11:07:59AM +0200, Peter Zijlstra wrote:
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/events/core.c |   65 ++++++++++++++++++++++++---------------------------
> >  1 file changed, 31 insertions(+), 34 deletions(-)
> > 
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -11285,49 +11285,46 @@ static void pmu_dev_release(struct devic
> >  
> >  static int pmu_dev_alloc(struct pmu *pmu)
> >  {
> > +	int ret;
> >  
> > +	struct device *dev __free(put_device) =
> > +		kzalloc(sizeof(struct device), GFP_KERNEL);
> > +	if (!dev)
> > +		return -ENOMEM;
> >  
> > +	dev->groups = pmu->attr_groups;
> > +	device_initialize(dev);
> >  
> > +	dev_set_drvdata(dev, pmu);
> > +	dev->bus = &pmu_bus;
> > +	dev->release = pmu_dev_release;
> >  
> > +	ret = dev_set_name(dev, "%s", pmu->name);
> >  	if (ret)
> > +		return ret;
> >  
> > +	ret = device_add(dev);
> >  	if (ret)
> > +		return ret;
> >  
> > +	struct device *del __free(device_del) = dev;
> 
> Greg, I'm not much familiar with the whole device model, but it seems
> unfortunate to me that one has to call device_del() explicitly if we
> already have a put_device() queued.
> 
> Is there a saner way to write this?

Ok, the "problem" here is that you have decided to do the "complex" way
to initialize a struct device.  And as such, you have to do more
housekeeping than if you were to just use the simple interface.

The rule is, after you call device_initialize() you HAVE to call
put_device() on the pointer if something goes wrong and you want to
clean up properly.  Unless you have called device_add(), and at that
point in time, then you HAVE to call device_del() if the device_add()
call succeeded.  If the device_add() call failed, then you HAVE to call
put_device().

Yeah, it's a pain, but you are trying to hand-roll code that is not a
"normal" path for a struct device, sorry.

I don't know if you really can encode all of that crazy logic in the
cleanup api, UNLESS you can "switch" the cleanup function at a point in
time (i.e. after device_add() is successful).  Is that possible?

Anyway, let me see about just cleaning up this code in general, I don't
think you need the complex interface here for a tiny struct device at
all, which would make this specific instance moot :)

Also, nit, you are racing with userspace by attempting to add new device
files _AFTER_ the device is registered with the driver core, this whole
thing can be made more simpler I hope, give me a bit...

thanks

greg k-h