diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/usb/URB.txt | 22 | ||||
| -rw-r--r-- | Documentation/usb/usbmon.txt | 6 |
2 files changed, 25 insertions, 3 deletions
diff --git a/Documentation/usb/URB.txt b/Documentation/usb/URB.txt index 8ffce746d49..00d2c644068 100644 --- a/Documentation/usb/URB.txt +++ b/Documentation/usb/URB.txt @@ -168,6 +168,28 @@ that if the completion handler or anyone else tries to resubmit it they will get a -EPERM error. Thus you can be sure that when usb_kill_urb() returns, the URB is totally idle. +There is a lifetime issue to consider. An URB may complete at any +time, and the completion handler may free the URB. If this happens +while usb_unlink_urb or usb_kill_urb is running, it will cause a +memory-access violation. The driver is responsible for avoiding this, +which often means some sort of lock will be needed to prevent the URB +from being deallocated while it is still in use. + +On the other hand, since usb_unlink_urb may end up calling the +completion handler, the handler must not take any lock that is held +when usb_unlink_urb is invoked. The general solution to this problem +is to increment the URB's reference count while holding the lock, then +drop the lock and call usb_unlink_urb or usb_kill_urb, and then +decrement the URB's reference count. You increment the reference +count by calling + + struct urb *usb_get_urb(struct urb *urb) + +(ignore the return value; it is the same as the argument) and +decrement the reference count by calling usb_free_urb. Of course, +none of this is necessary if there's no danger of the URB being freed +by the completion handler. + 1.7. What about the completion handler? diff --git a/Documentation/usb/usbmon.txt b/Documentation/usb/usbmon.txt index 5335fa8b06e..c42bb9cd3b4 100644 --- a/Documentation/usb/usbmon.txt +++ b/Documentation/usb/usbmon.txt @@ -183,10 +183,10 @@ An input control transfer to get a port status. d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 < d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000 -An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper -to a storage device at address 5: +An output bulk transfer to send a SCSI command 0x28 (READ_10) in a 31-byte +Bulk wrapper to a storage device at address 5: -dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000 +dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 ad000000 00800000 80010a28 20000000 20000040 00000000 000000 dd65f0e8 4128379808 C Bo:1:005:2 0 31 > * Raw binary format and API |