Hello, Bart, Mike. On Thu, Sep 06, 2012 at 08:52:09PM +0200, Bart Van Assche wrote: > >> The purpose of this patch is indeed to make *blk_run_queue() calls from > >> the block layer safe. There are several direct or indirect > >> *blk_run_queue() calls in the block layer where a reference on the queue > >> is held but not on the sdev, e.g. in the md, dm and bsg drivers. Yeah, while writing blk_drain_queue(), I was thinking only about requests. I didn't consider control reaching into driver. > > Is there a race still? If some blk code is calling blk_run_queue > > (waiting on the queue lock) but no IO is queued, > > blk_drain_queue/blk_cleanup_queue could complete since the drain > > counters are zero. Then blk_run_queue could grab the queue lock and call > > the request_fn on a freed scsi_device (sdev pointed to by q->queuedata > > would be freed so scsi_reuqest_fn would be freed). > > > > Do we need a check in __blk_run_queue for QUEUE_FLAG_DEAD (if dead then > > fail IO?), or do we need a check in scsi_request_fn for this. A dead > > queue check or maybe null q->queuedata in > > scsi_device_dev_release_usercontext (do this with the queue lock held), > > then check for null q->queuedata in scsi_request_fn? > > Yet another scenario is that scsi_remove_host() gets invoked and > finishes after scsi_request_fn() has unlocked the queue and before it > locks the queue again. That's a scenario that can't be handled by adding > more checks at the start of __blk_run_queue() or scsi_request_fn(). I think Mike is wondering whether your patch in isolation is enough or we also need to have DEAD check there too. The proposed patch can't handle the case where q->request_fn() is invoked after drain is complete. I'm not really sure whether that can happen tho. Other than that, yeah, I think it's a real problem and we definitely want to remove get/put_device() from scsi_request_fn(). Refcnting oneself is often wrong (as shown here too) and generally just sad. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
