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SubjectRe: [PATCH v4] driver core: hold dev's parent lock when needed
On Thu, 31 May 2018, Martin Liu wrote:

> SoC have internal I/O buses that can't be proved for devices. The
> devices on the buses can be accessed directly without additinal
> configuration required. This type of bus is represented as
> "simple-bus". In some platforms, we name "soc" with "simple-bus"
> attribute and many devices are hooked under it described in DT
> (device tree).
>
> In commit bf74ad5bc417 ("Hold the device's parent's lock during
> probe and remove") to solve USB subsystem lock sequence since
> USB device's characteristic. Thus "soc" needs to be locked
> whenever a device and driver's probing happen under "soc" bus.
> During this period, an async driver tries to probe a device which
> is under the "soc" bus would be blocked until previous driver
> finish the probing and release "soc" lock. And the next probing
> under the "soc" bus need to wait for async finish. Because of
> that, driver's async probe for init time improvement will be
> shadowed.
>
> Since many devices don't have USB devices' characteristic, they
> actually don't need parent's lock. Thus, we introduce a lock flag
> in bus_type struct and driver core would lock the parent lock base
> on the flag. For USB, we set this flag in usb_bus_type to keep
> original lock behavior in driver core.
>
> Async probe could have more benefit after this patch.
>
> Signed-off-by: Martin Liu <liumartin@google.com>
> ---
> Changes in v4:
> -fix comment and wording.
> -follow the suggestion.
>
> [v3]: https://lkml.org/lkml/2018/5/29/876
> [v2]: https://lkml.org/lkml/2018/5/29/108
> [v1]: https://lkml.org/lkml/2018/5/22/545
>
> drivers/base/bus.c | 16 ++++++++--------
> drivers/base/dd.c | 8 ++++----
> drivers/usb/core/driver.c | 1 +
> include/linux/device.h | 3 +++
> 4 files changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> index ef6183306b40..8bfd27ec73d6 100644
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -184,10 +184,10 @@ static ssize_t unbind_store(struct device_driver *drv, const char *buf,
>
> dev = bus_find_device_by_name(bus, NULL, buf);
> if (dev && dev->driver == drv) {
> - if (dev->parent) /* Needed for USB */
> + if (dev->parent && dev->bus->need_parent_lock)
> device_lock(dev->parent);
> device_release_driver(dev);
> - if (dev->parent)
> + if (dev->parent && dev->bus->need_parent_lock)
> device_unlock(dev->parent);
> err = count;
> }
> @@ -211,12 +211,12 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf,
>
> dev = bus_find_device_by_name(bus, NULL, buf);
> if (dev && dev->driver == NULL && driver_match_device(drv, dev)) {
> - if (dev->parent) /* Needed for USB */
> + if (dev->parent && bus->need_parent_lock)
> device_lock(dev->parent);
> device_lock(dev);
> err = driver_probe_device(drv, dev);
> device_unlock(dev);
> - if (dev->parent)
> + if (dev->parent && bus->need_parent_lock)
> device_unlock(dev->parent);
>
> if (err > 0) {
> @@ -735,10 +735,10 @@ static int __must_check bus_rescan_devices_helper(struct device *dev,
> int ret = 0;
>
> if (!dev->driver) {
> - if (dev->parent) /* Needed for USB */
> + if (dev->parent && dev->bus->need_parent_lock)
> device_lock(dev->parent);
> ret = device_attach(dev);
> - if (dev->parent)
> + if (dev->parent && dev->bus->need_parent_lock)
> device_unlock(dev->parent);
> }
> return ret < 0 ? ret : 0;
> @@ -770,10 +770,10 @@ EXPORT_SYMBOL_GPL(bus_rescan_devices);
> int device_reprobe(struct device *dev)
> {
> if (dev->driver) {
> - if (dev->parent) /* Needed for USB */
> + if (dev->parent && dev->bus->need_parent_lock)
> device_lock(dev->parent);
> device_release_driver(dev);
> - if (dev->parent)
> + if (dev->parent && dev->bus->need_parent_lock)
> device_unlock(dev->parent);
> }
> return bus_rescan_devices_helper(dev, NULL);
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index c9f54089429b..7c09f73b96f3 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -817,13 +817,13 @@ static int __driver_attach(struct device *dev, void *data)
> return ret;
> } /* ret > 0 means positive match */
>
> - if (dev->parent) /* Needed for USB */
> + if (dev->parent && dev->bus->need_parent_lock)
> device_lock(dev->parent);
> device_lock(dev);
> if (!dev->driver)
> driver_probe_device(drv, dev);
> device_unlock(dev);
> - if (dev->parent)
> + if (dev->parent && dev->bus->need_parent_lock)
> device_unlock(dev->parent);
>
> return 0;
> @@ -919,7 +919,7 @@ void device_release_driver_internal(struct device *dev,
> struct device_driver *drv,
> struct device *parent)
> {
> - if (parent)
> + if (parent && dev->bus->need_parent_lock)
> device_lock(parent);
>
> device_lock(dev);
> @@ -927,7 +927,7 @@ void device_release_driver_internal(struct device *dev,
> __device_release_driver(dev, parent);
>
> device_unlock(dev);
> - if (parent)
> + if (parent && dev->bus->need_parent_lock)
> device_unlock(parent);
> }
>
> diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
> index 9792cedfc351..e76e95f62f76 100644
> --- a/drivers/usb/core/driver.c
> +++ b/drivers/usb/core/driver.c
> @@ -1922,4 +1922,5 @@ struct bus_type usb_bus_type = {
> .name = "usb",
> .match = usb_device_match,
> .uevent = usb_uevent,
> + .need_parent_lock = true,
> };
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 477956990f5e..beca424395dd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -98,6 +98,8 @@ extern void bus_remove_file(struct bus_type *, struct bus_attribute *);
> * @lock_key: Lock class key for use by the lock validator
> * @force_dma: Assume devices on this bus should be set up by dma_configure()
> * even if DMA capability is not explicitly described by firmware.
> + * @need_parent_lock: When probing or removing a device on this bus, the
> + * device core should lock the device's parent.
> *
> * A bus is a channel between the processor and one or more devices. For the
> * purposes of the device model, all devices are connected via a bus, even if
> @@ -138,6 +140,7 @@ struct bus_type {
> struct lock_class_key lock_key;
>
> bool force_dma;
> + bool need_parent_lock;
> };
>
> extern int __must_check bus_register(struct bus_type *bus);

This looks okay to me.

Acked-by: Alan Stern <stern@rowland.harvard.edu>

Greg, any more comments?

Alan Stern

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