Greg:
This patch fixes a problem with my recent set of locking changes for USB.
The problem is that rw-semaphores don't have the semantics I need. I need
something where, if the semaphore is locked for reading and a writer is
waiting for the lock, another reader will be granted a readlock
immediately. That's because there are several places where a thread
holding the readlock will acquire the readlock again, in a nested or
recursive fashion. If a writer is waiting for the first readlock to be
released, the standard semantics will yield deadlock.
This patch implements those alternate semantics by putting writers on a
separate wait queue. It's a little bit awkward and has a definite
roll-you-own flavor, but it works. Please apply.
Alan Stern
Signed-off-by: Alan Stern <[EMAIL PROTECTED]>
===== drivers/usb/core/usb.c 1.281 vs edited =====
--- 1.281/drivers/usb/core/usb.c Wed Jun 30 09:44:26 2004
+++ edited/drivers/usb/core/usb.c Wed Jul 7 15:47:23 2004
@@ -64,6 +64,7 @@
/* Not honored on modular build */
static DECLARE_RWSEM(usb_all_devices_rwsem);
+static DECLARE_WAIT_QUEUE_HEAD(usb_all_devices_wqh);
static int generic_probe (struct device *dev)
@@ -933,6 +934,7 @@
{
up(&udev->serialize);
up_read(&usb_all_devices_rwsem);
+ wake_up(&usb_all_devices_wqh);
}
/**
@@ -940,10 +942,15 @@
*
* This is necessary when registering a new driver or probing a bus,
* since the driver-model core may try to use any usb_device.
+ *
+ * Unfortunately we have to use a separate wait queue, because we need
+ * to make sure that a thread waiting for a writelock won't block other
+ * threads from acquiring a readlock.
*/
void usb_lock_all_devices(void)
{
- down_write(&usb_all_devices_rwsem);
+ wait_event(usb_all_devices_wqh,
+ down_write_trylock(&usb_all_devices_rwsem));
}
/**
@@ -952,6 +959,7 @@
void usb_unlock_all_devices(void)
{
up_write(&usb_all_devices_rwsem);
+ wake_up(&usb_all_devices_wqh);
}
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