On 06/09/2015 11:01 AM, Matthew R. Ochs wrote: > Brian/Manoj, > > See my alternate proposal below. > > -matt > > 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? -Brian -- Brian King Power Linux I/O IBM Linux Technology Center -- 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
