On Jun 9, 2015, at 1:30 PM, Brian King wrote: > On 06/09/2015 11:01 AM, Matthew R. Ochs wrote: >> >> On Jun 9, 2015, at 9:37 AM, Manoj Kumar wrote: >>> On 6/9/2015 6:29 AM, Brian King wrote: >>>>> >>>>> This was the optimization to avoid the MMIO for both threads. The other >>>>> thread that raced should >>>>> do the atomic set of afu->room to a positive value. >>>> >>>> Let's take the simpler scenario of just one thread. >>>> >>>> Let's start with afu->room = 1 >>>> We call atomic64_dec_if_positive, which results in afu->room going to zero >>>> and 0 being returned, >>>> so we go into the if leg. >>>> >>>> If afu->room is zero every time we read it from the adapter and we exhaust >>>> our retries, >>>> we return SCSI_MLQUEUE_HOST_BUSY. However, the next time we enter >>>> cxlflash_send_cmd, >>>> since afu->cmd is now 0, it will no longer get decremented, but the return >>>> value will >>>> be -1, so we'll go down the else if leg. We'll never get into the if leg >>>> again to >>>> re-read afu->room from the AFU. The simplest fix might just be to set >>>> afu->room = 1 >>>> if you ever leave the if leg without having room. >>> >>> Good suggestion. Will atomic64_set(&afu->room, 1), if we exhaust retries in >>> both legs. >> >> While I agree this will work it seems a bit of a kludge. >> >> What if we instead take advantage of our existing work queue and create a >> new work item that >> simply MMIO reads and atomically sets afu->room? With this, instead of >> slamming in a 1 to >> satisfy our logic such that a subsequent command will MMIO read, we would >> schedule the new >> work item and let afu->room be updated with a real value from the card. >> >> The only downside I see with this approach is that it has the potential to >> relax the window of time >> that we're 'down' (no room) and sending back busy...although that might not >> be such a bad thing >> if we were to get into this condition. > > Seems reasonable. Would be simpler. I assume you then just schedule the work > from cxlflash_send_cmd and > just return SCSI_MLQUEUE_HOST_BUSY from cxlflash_send_cmd if you don't have > room?
Correct. We'll keep things similar to how they are now (try our best to get room on the queuecommand thread) and then if we're unable to get room (udelay/retries exhausted) we'll schedule the work and return the busy rc. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
