On Fri, 8 Nov 2013, Sebastian Andrzej Siewior wrote: > On 03.07.13, Ming Lei wrote: ... > > + /* > > + * We disable local IRQs here avoid possible deadlock because > > + * drivers may call spin_lock() to hold lock which might be > > + * acquired in one hard interrupt handler. > > + * > > + * The local_irq_save()/local_irq_restore() around complete() > > + * will be removed if current USB drivers have been cleaned up > > + * and no one may trigger the above deadlock situation when > > + * running complete() in tasklet. > > + */ > > + local_irq_save(flags); > > + urb->complete(urb); > > + local_irq_restore(flags); > > How can there be a dead lock if you always complete in the same context > (either BH or hardirq) for a given device [0]?
A lock will often be acquired in more than one place. For example, in both an URB completion handler and a timer routine. Suppose a driver does spin_lock_irq(&lock) in the timer handler, but does spin_lock(&lock) in the completion handler, expecting that the completion handler will always be called with interrupts disabled. If the completion handler now runs with interrupts enabled, this would lead to deadlock if a timer interrupt occurs while the completion handler is running. > Why do you choose to complete in BH context instead using threaded irqs > and complete in the thread? This has been discussed a few times already in the mailing list. Briefly, measurements showed noticeably decreased throughput for USB mass storage transfers using threaded IRQs, compared to the BH implementation. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
