linux-kernel - Re: [PATCH v8 2/2] x86/sgx: Add an attribute for the amount of SGX memory in a NUMA node

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Date:   Sun, 24 Oct 2021 17:24:43 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        reinette.chatre@...el.com, tony.luck@...el.com,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org
Subject: Re: [PATCH v8 2/2] x86/sgx: Add an attribute for the amount of SGX
 memory in a NUMA node

On Sat, 2021-10-23 at 08:33 +0200, Greg Kroah-Hartman wrote:
> On Sat, Oct 23, 2021 at 04:02:48AM +0300, Jarkko Sakkinen wrote:
> > On Mon, 2021-10-18 at 16:35 +0200, Greg Kroah-Hartman wrote:
> > > > +               ret = sysfs_create_group(&dev->kobj, &sgx_node_attr_group);
> > > 
> > > A huge hint, if a driver has to call a sysfs_* call, something is wrong.
> > > 
> > > Something is wrong here.
> > > 
> > > Why are you messing around with a kobject?  This is a device, that you
> > > control, you can just set the default attribute group for it and then
> > > the driver core will add and remove the sysfs group at the proper time,
> > > in the proper way.  Right now you are racing userspace and loosing.
> > > 
> > > Use the default group list, that is what it is there for.
> > 
> > I used sysfs_create_group() because node_devices is not owned by SGX
> > code. It is managed in drivers/base/node.c, and also initialized before
> > SGX.
> 
> Then that is broken, please do not use that device as your code does not
> "own" it.  Or fix the logic to be initialized earlier.

To get a synchronous initialization, I'd need to add the attributes as
part of this declaration:

static struct attribute *node_dev_attrs[] = {
	&dev_attr_cpumap.attr,
	&dev_attr_cpulist.attr,
	&dev_attr_meminfo.attr,
	&dev_attr_numastat.attr,
	&dev_attr_distance.attr,
	&dev_attr_vmstat.attr,
	NULL
};
ATTRIBUTE_GROUPS(node_dev);

That guarantees that the attribute exists at the time when the
node is created, e.g. in that sense this will fix the race with
uevent code.

However, up until sgx_init() has been completed, the attribute
will emit '0'.

If I change sgx_init() from device_initcall() to
core_initcall() (i.e. one before postcore_initcall(), can I
expect these to work:

* node_isset()
* node_set()
* num_possibles_nodes()
* numa_node_id()
* next_node_in()

?

Dave, perhaps you know this?

[*] register_node_type() and kobject_event_init() are postcore init
    calls, so this would be non-racy.

> thanks,
> 
> greg k-h

/Jarkko