Hi, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: > On Mon, 26 Mar 2018, Felipe Balbi wrote: > >> Mention that ->complete() should never be called from within >> usb_ep_queue(). >> >> Signed-off-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> >> --- >> drivers/usb/gadget/udc/core.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c >> index 50988b21a21b..842814bc0e4f 100644 >> --- a/drivers/usb/gadget/udc/core.c >> +++ b/drivers/usb/gadget/udc/core.c >> @@ -238,6 +238,9 @@ EXPORT_SYMBOL_GPL(usb_ep_free_request); >> * arranges to poll once per interval, and the gadget driver usually will >> * have queued some data to transfer at that time. >> * >> + * Note that @req's ->complete() callback must never be called from >> + * within usb_ep_queue() as that can create deadlock situations. >> + * > > I think this is highly questionable. Certainly it was not David > Brownell's original intention; his dummy-hcd driver will sometimes > give back a request from within usb_ep_queue() -- and I believe he > wrote it that way in order to emulate a feature of his net2280 driver. > > In this particular case, the problem is that a driver acquires a > spinlock in its complete() routine, but then it holds that same > spinlock while submitting a request. This is a bug; it should be fixed > in the driver. The spinlock should be dropped while the request is > submitted. I'm sure there are examples whether other drivers do this. usb_ep_queue() can be called from atomic, there's no explicit requirement that locks should be released. Either one case or the other should be made explicit. -- balbi
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