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    SubjectRe: [PATCH] usb: ulpi: don't register drivers if bus doesn't exist
    On Thu, 28 May 2015, Felipe Balbi wrote:

    > On Thu, May 28, 2015 at 11:09:38AM +0530, Sudip Mukherjee wrote:
    > > On Wed, May 27, 2015 at 08:21:16AM -0700, Greg KH wrote:
    > > > On Wed, May 27, 2015 at 11:16:34AM -0400, Alan Stern wrote:
    > > > > On Wed, 27 May 2015, Heikki Krogerus wrote:
    > > >
    > > > Maybe we need to test for this in the driver core, not allowing drivers
    > > > for busses that are not registered, that might solve the main problem
    > > > here. I'll try to look at it tonight.
    > > may i suggest something like this ?
    > > buildtest with allmodconfig and allyesconfig on x86_64.
    > > built and booted on x86.
    > >
    > >
    > > diff --git a/drivers/base/bus.c b/drivers/base/bus.c
    > > index 5005924..95cefa0 100644
    > > --- a/drivers/base/bus.c
    > > +++ b/drivers/base/bus.c
    > > @@ -943,6 +943,7 @@ int bus_register(struct bus_type *bus)
    > > if (retval)
    > > goto bus_groups_fail;
    > >
    > > + bus->registered = true;
    >
    > once set, it's never cleared.

    It's worse than that...

    > > pr_debug("bus: '%s': registered\n", bus->name);
    > > return 0;
    > >
    > > diff --git a/drivers/base/driver.c b/drivers/base/driver.c
    > > index 4eabfe2..1acae5b 100644
    > > --- a/drivers/base/driver.c
    > > +++ b/drivers/base/driver.c
    > > @@ -150,6 +150,11 @@ int driver_register(struct device_driver *drv)
    > > int ret;
    > > struct device_driver *other;
    > >
    > > + if (!drv->bus->registered) {
    > > + pr_err("Driver %s registration failed. bus not yet registered\n",
    > > + drv->name);
    > > + return -ENODEV;
    > > + }
    > > BUG_ON(!drv->bus->p);
    > >
    > > if ((drv->bus->probe && drv->probe) ||
    > > diff --git a/include/linux/device.h b/include/linux/device.h
    > > index 00ac57c..8fe4745 100644
    > > --- a/include/linux/device.h
    > > +++ b/include/linux/device.h
    > > @@ -126,6 +126,7 @@ struct bus_type {
    > > const struct dev_pm_ops *pm;
    > >
    > > const struct iommu_ops *iommu_ops;
    > > + bool registered; /* DON'T TOUCH THIS */
    >
    > I would rather use an atomic_t

    What reason is there to use an atomic_t? The value is never going to
    be changed by two threads at the same time.

    More importantly, clearing the flag races with checking it. If
    somebody tries to register a driver at the same time as the bus is
    unregistered, the result is undefined.

    Of course, the same problem exists when a device is added to a bus at
    the same time as the bus is unregistered.

    Alan Stern



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