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usb: core: Solve race condition in anchor cleanup functions
usb_kill_anchored_urbs() is commonly used to cancel all URBs on an anchor just before releasing resources which the URBs rely on. By doing so, users of this function rely on that no completer callbacks will take place from any URB on the anchor after it returns. However if this function is called in parallel with __usb_hcd_giveback_urb processing a URB on the anchor, the latter may call the completer callback after usb_kill_anchored_urbs() returns. This can lead to a kernel panic due to use after release of memory in interrupt context. The race condition is that __usb_hcd_giveback_urb() first unanchors the URB and then makes the completer callback. Such URB is hence invisible to usb_kill_anchored_urbs(), allowing it to return before the completer has been called, since the anchor's urb_list is empty. Even worse, if the racing completer callback resubmits the URB, it may remain in the system long after usb_kill_anchored_urbs() returns. Hence list_empty(&anchor->urb_list), which is used in the existing while-loop, doesn't reliably ensure that all URBs of the anchor are gone. A similar problem exists with usb_poison_anchored_urbs() and usb_scuttle_anchored_urbs(). This patch adds an external do-while loop, which ensures that all URBs are indeed handled before these three functions return. This change has no effect at all unless the race condition occurs, in which case the loop will busy-wait until the racing completer callback has finished. This is a rare condition, so the CPU waste of this spinning is negligible. The additional do-while loop relies on usb_anchor_check_wakeup(), which returns true iff the anchor list is empty, and there is no __usb_hcd_giveback_urb() in the system that is in the middle of the unanchor-before-complete phase. The @suspend_wakeups member of struct usb_anchor is used for this purpose, which was introduced to solve another problem which the same race condition causes, in commit 6ec4147e ("usb-anchor: Delay usb_wait_anchor_empty_timeout wake up till completion is done"). The surely_empty variable is necessary, because usb_anchor_check_wakeup() must be called with the lock held to prevent races. However the spinlock must be released and reacquired if the outer loop spins with an empty URB list while waiting for the unanchor-before-complete passage to finish: The completer callback may very well attempt to take the very same lock. To summarize, using usb_anchor_check_wakeup() means that the patched functions can return only when the anchor's list is empty, and there is no invisible URB being processed. Since the inner while loop finishes on the empty list condition, the new do-while loop will terminate as well, except for when the said race condition occurs. Signed-off-by:Eli Billauer <eli.billauer@gmail.com> Acked-by: