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:

quoted

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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