On 12/20/2011 09:56 PM, Jens Axboe wrote: > On 2011-12-20 02:45, Dan Williams wrote: >> On Mon, Dec 19, 2011 at 5:11 PM, Tao Ma <tm@xxxxxx> wrote: >>>>> Looks good, better than what we had. Applied. >>>> >>>> This appears to interact badly with scsi_adjust_queue_depth() when the >>>> tag space shrinks. I can reproduce a similar crash as reported in >>>> "3.2-rc2+git: kernel BUG at block/blk-core.c:1000! >>>> (__scsi_queue_insert)" [1]. >>>> >>>> I can hit "kernel BUG at block/blk-core.c:2268!" which is the same >>>> BUG_ON(blk_queued_rq(rq)) check reliably with: >>>> # for i in $(seq 0 10); do dd if=/dev/zero of=/dev/sdX & done >>>> # echo 4 > /sys/class/block/sdX/device/queue_depth >>>> >>>> The following fixes it for me, if this looks ok (versus reverting >>>> commit 5e081591) I'll roll it into a formal patch with Ed and Meelis' >>>> Reported-by. >>> Interesting. If I read the code correctly, real_max_depth is the maximum >>> queue depth we ever have and max_depth is the current depth. >>> >>> In your fix, we never resize the tag size to be smaller than max_depth. >>> So I think this patch does expose some problem, but not lead to the BUG. >> >> Yes, if we keep the "if (unlikely(tag >= bqt->max_depth))" check in >> blk_queue_end_tag() then the side effect is that we can never shrink >> the tag depth, which I don't think was intended. >> >>> And in your new comment, you mentioned that "request between new_depth >>> and max_depth can be in-flight", but max_depth <= real_max_depth, so >>> what's wrong with the comment? Sorry, but am I missing something here? >> >> Prior to the change blk_queue_end_tag() would continue to complete >> requests with a tag > max_depth, now it silently drops them on the >> floor leaving BUG_ON(blk_queued_rq(rq)) to trigger when we try to end >> the request Oh, I see. So maybe we should modify blk_queue_end_tag instead of it? > > Yeah, that' is just wrong. Tao Ma, which bug was the original fix intended > to fix? uh, actually there is no original bug for it. > > The reason we have these two ceilings is exactly for shrinking depth > situations. It's quite legal to have an inflight request with a tag > inbetween max_depth and real_max_depth. yeah, so I guess we should fix that in blk_queue_end_tag instead of reverting this check. Thanks Tao -- 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
