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

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Message-ID: <2023061226-effective-droplet-1ba3@gregkh>
Date:   Mon, 12 Jun 2023 14:58:55 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 02:29:05PM +0200, Paolo Bonzini wrote:
> On 6/12/23 14:18, Greg KH wrote:
> > 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?
> 
> What _could_ make sense is that device_add() completely takes ownership of
> the given pointer, and takes care of calling put_device() on failure.

I think we tried that decades ago :)

Problem is that the caller wants to clean stuff up associted with that
struct device-embedded structure _before_ the final put_device() is
called which would usually free the memory of the overall structure.

So device_add() can't call put_device() on the error path within it, you
need the caller to unwind it's local stuff before that happens.

> Then you can have
> 
> 	struct device *dev_struct __free(put_device) =
> 		kzalloc(sizeof(struct device), GFP_KERNEL);
> 
> 	struct device *dev __free(device_del) =
> 		device_add(no_free_ptr(dev_struct));
> 
> 	/* dev_struct is NULL now */
> 
> 	pmu->dev = no_free_ptr(dev);

I don't see how that works properly, what am I missing?

thanks,

greg k-h