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Subject[PATCH v6] usb: ohci: Proper handling of ed_rm_list to handle race condition between usb_kill_urb() and finish_unlinks()
From: AMAN DEEP <>

There is a race condition between finish_unlinks->finish_urb() function
and usb_kill_urb() in ohci controller case. The finish_urb calls
spin_unlock(&ohci->lock) before usb_hcd_giveback_urb() function call,
then if during this time, usb_kill_urb is called for another endpoint,
then new ed will be added to ed_rm_list at beginning for unlink, and
ed_rm_list will point to newly added.

When finish_urb() is completed in finish_unlinks() and ed->td_list
becomes empty as in below code (in finish_unlinks() function):

if (list_empty(&ed->td_list)) {
*last = ed->ed_next;
ed->ed_next = NULL;
} else if (ohci->rh_state == OHCI_RH_RUNNING) {
*last = ed->ed_next;
ed->ed_next = NULL;
ed_schedule(ohci, ed);

The *last = ed->ed_next will make ed_rm_list to point to ed->ed_next
and previously added ed by usb_kill_urb will be left unreferenced by
ed_rm_list. This causes usb_kill_urb() hang forever waiting for
finish_unlink to remove added ed from ed_rm_list.

The main reason for hang in this race condtion is addition and removal
of ed from ed_rm_list in the beginning during usb_kill_urb and later
last* is modified in finish_unlinks().

As suggested by Alan Stern, the solution for proper handling of
ohci->ed_rm_list is to remove ed from the ed_rm_list before finishing
any URBs. Then at the end, we can add ed back to the list if necessary.

This properly handle the updated ohci->ed_rm_list in usb_kill_urb().

Fixes:977dcfdc6031("USB:OHCI:don't lose track of EDs when a controller dies")
Acked-by: Alan Stern <>
CC: <>
Signed-off-by: Aman Deep <>
Signed-off-by: Jeffy Chen <>

Changes in v6:
This is a resend of Aman Deep's v5 patch [0], which solved the hang we
hit [1]. (Thanks Aman :)

The v5 has some format issues, so i slightly adjust the commit message.


drivers/usb/host/ohci-q.c | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/drivers/usb/host/ohci-q.c b/drivers/usb/host/ohci-q.c
index b2ec8c399363..4ccb85a67bb3 100644
--- a/drivers/usb/host/ohci-q.c
+++ b/drivers/usb/host/ohci-q.c
@@ -1019,6 +1019,8 @@ static void finish_unlinks(struct ohci_hcd *ohci)
* have modified this list. normally it's just prepending
* entries (which we'd ignore), but paranoia won't hurt.
+ *last = ed->ed_next;
+ ed->ed_next = NULL;
modified = 0;

/* unlink urbs as requested, but rescan the list after
@@ -1077,21 +1079,22 @@ static void finish_unlinks(struct ohci_hcd *ohci)
goto rescan_this;

- * If no TDs are queued, take ED off the ed_rm_list.
+ * If no TDs are queued, ED is now idle.
* Otherwise, if the HC is running, reschedule.
- * If not, leave it on the list for further dequeues.
+ * If the HC isn't running, add ED back to the
+ * start of the list for later processing.
if (list_empty(&ed->td_list)) {
- *last = ed->ed_next;
- ed->ed_next = NULL;
ed->state = ED_IDLE;
} else if (ohci->rh_state == OHCI_RH_RUNNING) {
- *last = ed->ed_next;
- ed->ed_next = NULL;
ed_schedule(ohci, ed);
} else {
- last = &ed->ed_next;
+ ed->ed_next = ohci->ed_rm_list;
+ ohci->ed_rm_list = ed;
+ /* Don't loop on the same ED */
+ if (last == &ohci->ed_rm_list)
+ last = &ed->ed_next;

if (modified)

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