On Fri, Aug 09, 2013 at 01:17:37PM -0700, Nicholas A. Bellinger wrote: > On Fri, 2013-08-09 at 21:15 +0200, Alexander Gordeev wrote: > Mmmm, I'm able to reproduce over here with ahci + scsi-mq, and it > appears to be a bug related with using sdev->sdev_md_req.queue_depth=1, > that ends up causing the blkdev_issue_flush() to wait forever because > blk_mq_wait_for_tags() never ends up getting the single tag back for the > WRITE_FLUSH bio -> SYNCHRONIZE_CACHE cdb. It turns out this way - blkdev_issue_flush() claims the only tag, submits the bio and waits for the completion. But because blk_mq_make_request() does not mark any context in blk_mq_hw_ctx::ctx_map (nor enslists the request into blk_mq_ctx::rq_list) it never gets processed from blk_mq_work_fn-> __blk_mq_run_hw_queue() and blkdev_issue_flush() waits endlessly. All other requests are just waiting for the tag availability as result. [...] > Bumping queue_depth=2 seems to work-around the issue, but AFAICT it's a > genuine tag starvation bug with queue_depth=1 and WRITE_FLUSH.. If I try to hack and force __blk_mq_run_hw_queue() to process the request... diff --git a/block/blk-mq.c b/block/blk-mq.c index 6fc1df3..c22b6f66 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -889,9 +962,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio) hctx->queued++; if (unlikely(is_flush_fua)) { + list_add(&rq->queuelist, &hctx->dispatch); blk_mq_bio_to_request(q, rq, bio); blk_mq_put_ctx(ctx); blk_insert_flush(rq); goto run_queue; } ... I get a kernel BUG at drivers/scsi/scsi_lib.c:1233 BUG_ON(!req->nr_phys_segments); IOW I am not sure how to proceed. > --nab > -- Regards, Alexander Gordeev agordeev@xxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html