Il 04/09/2012 16:19, Michael S. Tsirkin ha scritto: > > > Also - some kind of comment explaining why a similar race can not happen > > > with this lock in place would be nice: I see why this specific race can > > > not trigger but since lock is dropped later before you submit command, I > > > have hard time convincing myself what exactly gurantees that vq is never > > > switched before or even while command is submitted. > > > > Because tgt->reqs will never become zero (which is a necessary condition > > for tgt->req_vq to change), as long as one request is executing > > virtscsi_queuecommand. > > Yes but this logic would apparently imply the lock is not necessary, and > it actually is. I am not saying anything is wrong just that it > looks scary.
Ok, I get the misunderstanding. For the logic to hold, you need a serialization point after which tgt->req_vq is not changed. The lock provides one such serialization point: after you unlock tgt->tgt_lock, nothing else will change tgt->req_vq until your request completes. Without the lock, there could always be a thread that is in the "then" branch but has been scheduled out, and when rescheduled it will change tgt->req_vq. Perhaps the confusion comes from the atomic_inc_return, and that was what my "why is this atomic" wanted to clear. **tgt->reqs is only atomic to avoid taking a spinlock in the ISR**. If you read the code with the lock, but with tgt->reqs as a regular non-atomic int, it should be much easier to reason on the code. I can split the patch if needed. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
