linux-kernel - Re: [RFC][PATCH V3] axi: add AXI bus driver

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Date:	Tue, 12 Apr 2011 00:45:33 +0200
From:	Rafał Miłecki <zajec5@...il.com>
To:	Greg KH <greg@...ah.com>
Cc:	linux-wireless@...r.kernel.org,
	"John W. Linville" <linville@...driver.com>,
	George Kashperko <george@...u.edu.ua>,
	Arnd Bergmann <arnd@...db.de>,
	Russell King <rmk@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	b43-dev@...ts.infradead.org,
	Michael Büsch <mb@...sch.de>,
	linuxdriverproject <devel@...uxdriverproject.org>,
	Andy Botting <andy@...ybotting.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Larry Finger <Larry.Finger@...inger.net>
Subject: Re: [RFC][PATCH V3] axi: add AXI bus driver

2011/4/12 Greg KH <greg@...ah.com>:
> On Tue, Apr 12, 2011 at 12:12:47AM +0200, Rafał Miłecki wrote:
>> 2011/4/11 Greg KH <greg@...ah.com>:
>> > On Mon, Apr 11, 2011 at 11:36:39PM +0200, Rafał Miłecki wrote:
>> >> 2011/4/11 Greg KH <greg@...ah.com>:
>> >> > Please read the documentation for how to do this properly.  I find it
>> >> > really hard to believe that you wrote that comment instead of putting in
>> >> > the 2 lines of code required for this function.
>> >> >
>> >> > Especially as-it-is, your code does not work properly and leaks memory
>> >> > badly.  Why would you do that on purpose?
>> >>
>> >> I tried to read some documentation about this.
>> >>
>> >> 1) driver-mode/device.txt says only that:
>> >> > Callback to free the device after all references have
>> >> > gone away. This should be set by the allocator of the
>> >> > device (i.e. the bus driver that discovered the device).
>> >> I *really* do not know how my driver should "free" core on AXI bus.
>> >
>> > The structure that you have created, added to the bus, is now ready to
>> > have its memory freed.  So free it.
>> >
>> > This usually means something like:
>> >        struct my_obj = to_my_obj(dev);
>> >        kfree(my_obj);
>> > in the release function.
>>
>> I register core->dev to the bus (I set core->dev.bus and
>> core->dev.parent, is that what you mean?). This core->dev is "struct
>> dev" embedded in "struct axi_device". By embedded I mean it is *not* a
>> pointer, I do not alloc it, it's part of the "struct axi_device".
>
> That is exactly as it should be.
>
> Then in your release function, free the struct axi_device.  It's that
> simple.  To try to free it before then would be wrong and cause
> problems.

This is because it is defined as:
struct axi_device cores[AXI_MAX_NR_CORES];


>> >> 4) SSB in it's ssb_release_dev just calls kfree on struct that was
>> >> allocated when registering drivers. *I do not* allocate such a struct,
>> >> so I believe I do exactly the same memory leak as SSB does.
>> >
>> > Well someone allocated it, right?  Who did it?  If it wasn't you, where
>> > did that structure come from and why are you registering it on your bus?
>> >
>> >> Can you spend 2 more minues in addition to commenting my ideas and
>> >> help me with writing that 2 lines I missed? Where do I leak memory in
>> >> my driver? Which struct should I kfree?
>> >
>> > The structure that you wrap around 'struct device' for your bus.
>>
>> As explained above, this I do not dynamically alloc this 'struct
>> device'. So is there really any memory leak?
>
> Yes, no one ever freed your struct axi_device that you created.

I agree that defining "cores" as array with maxium possible size
instead of linked list eats more memory than needed, but at least it
does not leak anything.

-- 
Rafał
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