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[PATCH] USB: Remove USB private semaphore

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This patch (as605) removes the private udev->serialize semaphore,
relying instead on the locking provided by the embedded struct device's
semaphore.  The changes are confined to the core, except that the
usb_trylock_device routine now uses the return convention of
down_trylock rather than down_read_trylock (they return opposite values
for no good reason).

A couple of other associated changes are included as well:

	Now that we aren't concerned about HCDs that avoid using the
	hcd glue layer, usb_disconnect no longer needs to acquire the
	usb_bus_lock -- that can be done by usb_remove_hcd where it
	belongs.

	Devices aren't locked over the same scope of code in
	usb_new_device and hub_port_connect_change as they used to be.
	This shouldn't cause any trouble.

Along with the preceding driver core patch, this needs a lot of testing.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Alan Stern
2005-11-17 17:10:32 -05:00
committed by Greg Kroah-Hartman
parent 75318d2d7c
commit 9ad3d6ccf5
9 changed files with 37 additions and 154 deletions
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114
drivers/usb/core/usb.c
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@@ -32,7 +32,6 @@
#include <linux/spinlock.h>
#include <linux/errno.h>
#include <linux/smp_lock.h>
#include <linux/rwsem.h>
#include <linux/usb.h>
#include <asm/io.h>
@@ -49,8 +48,6 @@ const char *usbcore_name = "usbcore";
static int nousb; /* Disable USB when built into kernel image */
/* Not honored on modular build */
static DECLARE_RWSEM(usb_all_devices_rwsem);
/**
* usb_ifnum_to_if - get the interface object with a given interface number
@@ -446,8 +443,6 @@ usb_alloc_dev(struct usb_device *parent, struct usb_bus *bus, unsigned port1)
dev->parent = parent;
INIT_LIST_HEAD(&dev->filelist);
init_MUTEX(&dev->serialize);
return dev;
}
@@ -520,75 +515,20 @@ void usb_put_intf(struct usb_interface *intf)
/* USB device locking
*
* Although locking USB devices should be straightforward, it is
* complicated by the way the driver-model core works. When a new USB
* driver is registered or unregistered, the core will automatically
* probe or disconnect all matching interfaces on all USB devices while
* holding the USB subsystem writelock. There's no good way for us to
* tell which devices will be used or to lock them beforehand; our only
* option is to effectively lock all the USB devices.
*
* We do that by using a private rw-semaphore, usb_all_devices_rwsem.
* When locking an individual device you must first acquire the rwsem's
* readlock. When a driver is registered or unregistered the writelock
* must be held. These actions are encapsulated in the subroutines
* below, so all a driver needs to do is call usb_lock_device() and
* usb_unlock_device().
* USB devices and interfaces are locked using the semaphore in their
* embedded struct device. The hub driver guarantees that whenever a
* device is connected or disconnected, drivers are called with the
* USB device locked as well as their particular interface.
*
* Complications arise when several devices are to be locked at the same
* time. Only hub-aware drivers that are part of usbcore ever have to
* do this; nobody else needs to worry about it. The problem is that
* usb_lock_device() must not be called to lock a second device since it
* would acquire the rwsem's readlock reentrantly, leading to deadlock if
* another thread was waiting for the writelock. The solution is simple:
*
* When locking more than one device, call usb_lock_device()
* to lock the first one. Lock the others by calling
* down(&udev->serialize) directly.
*
* When unlocking multiple devices, use up(&udev->serialize)
* to unlock all but the last one. Unlock the last one by
* calling usb_unlock_device().
* do this; nobody else needs to worry about it. The rule for locking
* is simple:
*
* When locking both a device and its parent, always lock the
* the parent first.
*/
/**
* usb_lock_device - acquire the lock for a usb device structure
* @udev: device that's being locked
*
* Use this routine when you don't hold any other device locks;
* to acquire nested inner locks call down(&udev->serialize) directly.
* This is necessary for proper interaction with usb_lock_all_devices().
*/
void usb_lock_device(struct usb_device *udev)
{
down_read(&usb_all_devices_rwsem);
down(&udev->serialize);
}
/**
* usb_trylock_device - attempt to acquire the lock for a usb device structure
* @udev: device that's being locked
*
* Don't use this routine if you already hold a device lock;
* use down_trylock(&udev->serialize) instead.
* This is necessary for proper interaction with usb_lock_all_devices().
*
* Returns 1 if successful, 0 if contention.
*/
int usb_trylock_device(struct usb_device *udev)
{
if (!down_read_trylock(&usb_all_devices_rwsem))
return 0;
if (down_trylock(&udev->serialize)) {
up_read(&usb_all_devices_rwsem);
return 0;
}
return 1;
}
/**
* usb_lock_device_for_reset - cautiously acquire the lock for a
* usb device structure
@@ -627,7 +567,7 @@ int usb_lock_device_for_reset(struct usb_device *udev,
}
}
while (!usb_trylock_device(udev)) {
while (usb_trylock_device(udev) != 0) {
/* If we can't acquire the lock after waiting one second,
* we're probably deadlocked */
@@ -645,39 +585,6 @@ int usb_lock_device_for_reset(struct usb_device *udev,
return 1;
}
/**
* usb_unlock_device - release the lock for a usb device structure
* @udev: device that's being unlocked
*
* Use this routine when releasing the only device lock you hold;
* to release inner nested locks call up(&udev->serialize) directly.
* This is necessary for proper interaction with usb_lock_all_devices().
*/
void usb_unlock_device(struct usb_device *udev)
{
up(&udev->serialize);
up_read(&usb_all_devices_rwsem);
}
/**
* usb_lock_all_devices - acquire the lock for all usb device structures
*
* This is necessary when registering a new driver or probing a bus,
* since the driver-model core may try to use any usb_device.
*/
void usb_lock_all_devices(void)
{
down_write(&usb_all_devices_rwsem);
}
/**
* usb_unlock_all_devices - release the lock for all usb device structures
*/
void usb_unlock_all_devices(void)
{
up_write(&usb_all_devices_rwsem);
}
static struct usb_device *match_device(struct usb_device *dev,
u16 vendor_id, u16 product_id)
@@ -700,10 +607,10 @@ static struct usb_device *match_device(struct usb_device *dev,
/* look through all of the children of this device */
for (child = 0; child < dev->maxchild; ++child) {
if (dev->children[child]) {
down(&dev->children[child]->serialize);
usb_lock_device(dev->children[child]);
ret_dev = match_device(dev->children[child],
vendor_id, product_id);
up(&dev->children[child]->serialize);
usb_unlock_device(dev->children[child]);
if (ret_dev)
goto exit;
}
@@ -1300,10 +1207,7 @@ EXPORT_SYMBOL(usb_put_dev);
EXPORT_SYMBOL(usb_get_dev);
EXPORT_SYMBOL(usb_hub_tt_clear_buffer);
EXPORT_SYMBOL(usb_lock_device);
EXPORT_SYMBOL(usb_trylock_device);
EXPORT_SYMBOL(usb_lock_device_for_reset);
EXPORT_SYMBOL(usb_unlock_device);
EXPORT_SYMBOL(usb_driver_claim_interface);
EXPORT_SYMBOL(usb_driver_release_interface);