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USB documentation: explain lifetime rules for unlinking URBs

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This patch (as1534c) updates the documentation for usb_unlink_urb and
related functions.  It explains that the caller must prevent the URB
being unlinked from getting deallocated while the unlink is taking
place.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Alan Stern 2012-03-28 16:13:28 -04:00 committed by Greg Kroah-Hartman
parent bcf3985376
commit da8bfb090c
2 changed files with 34 additions and 0 deletions
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22
Documentation/usb/URB.txt
View File

@ -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?

12
drivers/usb/core/urb.c
View File

@ -539,6 +539,10 @@ EXPORT_SYMBOL_GPL(usb_submit_urb);
* never submitted, or it was unlinked before, or the hardware is already
* finished with it), even if the completion handler has not yet run.
*
* The URB must not be deallocated while this routine is running. In
* particular, when a driver calls this routine, it must insure that the
* completion handler cannot deallocate the URB.
*
* Unlinking and Endpoint Queues:
*
* [The behaviors and guarantees described below do not apply to virtual
@ -603,6 +607,10 @@ EXPORT_SYMBOL_GPL(usb_unlink_urb);
* with error -EPERM. Thus even if the URB's completion handler always
* tries to resubmit, it will not succeed and the URB will become idle.
*
* The URB must not be deallocated while this routine is running. In
* particular, when a driver calls this routine, it must insure that the
* completion handler cannot deallocate the URB.
*
* This routine may not be used in an interrupt context (such as a bottom
* half or a completion handler), or when holding a spinlock, or in other
* situations where the caller can't schedule().
@ -640,6 +648,10 @@ EXPORT_SYMBOL_GPL(usb_kill_urb);
* with error -EPERM. Thus even if the URB's completion handler always
* tries to resubmit, it will not succeed and the URB will become idle.
*
* The URB must not be deallocated while this routine is running. In
* particular, when a driver calls this routine, it must insure that the
* completion handler cannot deallocate the URB.
*
* This routine may not be used in an interrupt context (such as a bottom
* half or a completion handler), or when holding a spinlock, or in other
* situations where the caller can't schedule().