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SubjectRe: [RFC PATCH 1/7] drivers: base: Add resource managed version of delayed work init
On Sat, Feb 13, 2021 at 02:18:06PM +0100, Hans de Goede wrote:
> Hi,
>
> On 2/13/21 1:16 PM, Greg Kroah-Hartman wrote:
> > On Sat, Feb 13, 2021 at 01:58:44PM +0200, Matti Vaittinen wrote:
> >> A few drivers which need a delayed work-queue must cancel work at exit.
> >> Some of those implement remove solely for this purpose. Help drivers
> >> to avoid unnecessary remove and error-branch implementation by adding
> >> managed verision of delayed work initialization
> >>
> >> Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
> >
> > That's not a good idea. As this would kick in when the device is
> > removed from the system, not when it is unbound from the driver, right?
>
> Erm, no devm managed resources get released when the driver is detached:
> drivers/base/dd.c: __device_release_driver() calls devres_release_all(dev);

Then why do you have to manually call devm_free_irq() in release
callbacks? I thought that was the primary problem with those things.

I can understand devm_ calls handling resources, but callbacks and
workqueues feels like a big stretch.

> > There is two different lifespans here (well 3). Code and data*2. Don't
> > confuse them as that will just cause lots of problems.
> >
> > The move toward more and more "devm" functions is not the way to go as
> > they just more and more make things easier to get wrong.
> >
> > APIs should be impossible to get wrong, this one is going to be almost
> > impossible to get right.
>
> I have to disagree here devm generally makes it easier to get things right,
> it is when some devm functions are missing and devm and non devm resources
> are mixed that things get tricky.
>
> Lets look for example at the drivers/extcon/extcon-intel-int3496.c code
> from patch 2/7 from this set. The removed driver-remove function looks like
> this:
>
> -static int int3496_remove(struct platform_device *pdev)
> -{
> - struct int3496_data *data = platform_get_drvdata(pdev);
> -
> - devm_free_irq(&pdev->dev, data->usb_id_irq, data);
> - cancel_delayed_work_sync(&data->work);
> -
> - return 0;
> -}
> -
>
> This is a good example where the mix of devm and non devm (the workqueue)
> resources makes things tricky. The IRQ must be freed first to avoid the
> work potentially getting re-queued after the sync cancel.
>
> In this case using devm for the IRQ may cause the driver author to forget
> about this, leaving a race.
>
> Bit with the new proposed devm_delayed_work_autocancel() function things
> will just work.
>
> This work gets queued by the IRQ handler, so the work must be initialized (1)
> *before* devm_request_irq() gets called. Any different order would be a
> bug in the probe function since then the IRQ might run before the work
> is initialized.

How are we now going to audit the order of these calls to ensure that
this is done correctly? That still feels like it is ripe for bugs in a
much easier way than without these functions.

> Since devm unrolls / releases resources in reverse order, this means that
> it will automatically free the IRQ (which was requested later) before
> cancelling the work.
>
> So by switching to the new devm_delayed_work_autocancel() function we avoid
> a case where a driver author can cause a race on driver detach because it is
> relying on devm to free the IRQ, which may cause it to requeue a just
> cancelled work.
>
> IOW introducing this function (and using it where appropriate) actually
> removes a possible class of bugs.
>
> patch 2/7 actually has a nice example of this, drivers/extcon/extcon-gpio.c
> also uses a delayed work queued by an interrupt, together with devm managing
> the interrupt, yet the removed driver_remove callback:
>
> -static int gpio_extcon_remove(struct platform_device *pdev)
> -{
> - struct gpio_extcon_data *data = platform_get_drvdata(pdev);
> -
> - cancel_delayed_work_sync(&data->work);
> -
> - return 0;
> -}
> -
>
> Is missing the explicit free on the IRQ which is necessary to avoid
> the race. One the one hand this illustrates your (Greg's) argument that
> devm managed IRQs may be a bad idea.

I still think it is :)

> OTOH it shows that if we have devm managed IRQs anyways that then also
> having devm managed autocancel works is a good idea, since this RFC patch-set
> not only results in some cleanup, but is actually fixing at least 1 driver
> detach race condition.

Fixing bugs is good, but the abstraction away from resource management
that the devm_ calls cause is worrying as the "magic" behind them can be
wrong, as seen here.

thanks,

greg k-h

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