Re: [driver-core PATCH v8 2/9] driver core: Establish order of operations for device_add and device_del via bitflag

[Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Mon, 2018-12-10 at 11:43 -0800, Dan Williams wrote:
> On Mon, Dec 10, 2018 at 11:35 AM Alexander Duyck
> <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> wrote:
> > 
> > On Mon, 2018-12-10 at 10:58 -0800, Dan Williams wrote:
> > > On Wed, Dec 5, 2018 at 9:25 AM Alexander Duyck
> > > <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> wrote:
> > > > 
> > > > Add an additional bit flag to the device struct named "dead".
> > > > 
> > > > This additional flag provides a guarantee that when a device_del is
> > > > executed on a given interface an async worker will not attempt to attach
> > > > the driver following the earlier device_del call. Previously this
> > > > guarantee was not present and could result in the device_del call
> > > > attempting to remove a driver from an interface only to have the async
> > > > worker attempt to probe the driver later when it finally completes the
> > > > asynchronous probe call.
> > > > 
> > > > Signed-off-by: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> > > > ---
> > > >  drivers/base/core.c    |   11 +++++++++++
> > > >  drivers/base/dd.c      |    8 ++++++--
> > > >  include/linux/device.h |    5 +++++
> > > >  3 files changed, 22 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > > index f3e6ca4170b4..70358327303b 100644
> > > > --- a/drivers/base/core.c
> > > > +++ b/drivers/base/core.c
> > > > @@ -2075,6 +2075,17 @@ void device_del(struct device *dev)
> > > >         struct kobject *glue_dir = NULL;
> > > >         struct class_interface *class_intf;
> > > > 
> > > > +       /*
> > > > +        * Hold the device lock and set the "dead" flag to guarantee that
> > > > +        * the update behavior is consistent with the other bitfields near
> > > > +        * it and that we cannot have an asynchronous probe routine trying
> > > > +        * to run while we are tearing out the bus/class/sysfs from
> > > > +        * underneath the device.
> > > > +        */
> > > > +       device_lock(dev);
> > > > +       dev->dead = true;
> > > > +       device_unlock(dev);
> > > > +
> > > >         /* Notify clients of device removal.  This call must come
> > > >          * before dpm_sysfs_remove().
> > > >          */
> > > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > > index 88713f182086..3bb8c3e0f3da 100644
> > > > --- a/drivers/base/dd.c
> > > > +++ b/drivers/base/dd.c
> > > > @@ -774,6 +774,10 @@ static void __device_attach_async_helper(void *_dev, async_cookie_t cookie)
> > > > 
> > > >         device_lock(dev);
> > > > 
> > > > +       /* device is or has been removed from the bus, just bail out */
> > > > +       if (dev->dead)
> > > > +               goto out_unlock;
> > > > +
> > > 
> > > What do you think about moving this check into
> > > __device_attach_driver() alongside all the other checks? That way we
> > > also get ->dead checking through the __device_attach() path.
> > 
> > I'm not really sure that is the best spot to do that. Part of the
> > reason being that by placing it where I did we avoid messing with the
> > runtime power management for the parent if it was already powered off.
> 
> ...but this already a rare event and the parent shouldn't otherwise be
> bothered by a spurious pm_runtime wakeup event.
> 
> > If anything I would say we could probably look at pulling the check out
> > and placing the driver check in __device_attach_async_helper since from
> > what I can tell the check is actually redundant in the non-async path
> > anyway since __device_attach already had taken the device lock and
> > checked dev->driver prior to calling __device_attach_driver.
> > 
> > > ...and after that maybe it could be made a common helper
> > > (dev_driver_checks()?) shared between __device_attach_driver() and
> > > __driver_attach() to reduce some duplication.
> > 
> > I'm not sure consolidating it into a function would really be worth the
> > extra effort. It would essentially just obfuscate the checks and I am
> > not sure you really save much with:
> >         if (dev_driver_checks(dev))
> > vs:
> >         if (!dev->dead && !dev->driver)
> > 
> > By the time you create the function and replace the few spots that are
> > making these checks you would end up most likely adding more complexity
> > to the kernel rather than reducing it any.
> 
> No, I was talking about removing this duplication in
> __device_attach_driver() and __driver_attach():
> 
>         if (ret == 0) {
>                 /* no match */
>                 return 0;
>         } else if (ret == -EPROBE_DEFER) {
>                 dev_dbg(dev, "Device match requests probe deferral\n");
>                 driver_deferred_probe_add(dev);

Is this bit of code correct? Seems like there should be a return here
doesn't it?

I just double checked and this is what is in the kernel too.

>         } else if (ret < 0) {
>                 dev_dbg(dev, "Bus failed to match device: %d", ret);
>                 return ret;
>         } /* ret > 0 means positive match */
> 
> ...and lead in with a dev->dead check.

I would think that we would want to check for dev->dead before we even
call driver_match_device. That way we don't have the match function
crawling around a device that is being disassembled. Is that what you
were referring to?

Also the context for the two functions seems to be a bit different. In
the case of __device_attach_driver the device_lock is already held. In
__driver_attach the lock on the device isn't taken until after a match
has been found.