On Tue, Jan 25 2011 at  3:46pm -0500,
Vivek Goyal [off-list ref] wrote:

On Sun, Jan 23, 2011 at 11:31:33AM +0100, Tejun Heo wrote:

quoted

On Fri, Jan 21, 2011 at 07:49:55PM -0500, Mike Snitzer wrote:

quoted

quoted

+ * If the device doesn't have writeback cache, FLUSH and FUA don't make any
+ * difference.  The requests are either completed immediately if there's no
+ * data or executed as normal requests otherwise.

For devices without a writeback cache, I'm not seeing where pure flushes
are completed immediately.  But I do see where data is processed
directly in blk_insert_flush().

Yeah, it does.  Pure flushes on a device w/o writeback cache, @policy
is zero and blk_flush_complete_seq() will directly proceed to
REQ_FSEQ_DONE.

I see following code in __generic_make_request(). I am wondering if empty
flushes will be completed here itself if device does not have writeback
cache.

                /*
                 * Filter flush bio's early so that make_request based
                 * drivers without flush support don't have to worry
                 * about them.
                 */
                if ((bio->bi_rw & (REQ_FLUSH | REQ_FUA)) && !q->flush_flags) {
                        bio->bi_rw &= ~(REQ_FLUSH | REQ_FUA);
                        if (!nr_sectors) {
                                err = 0;
                                goto end_io;
                        }
                }

Yes, due to the !q->flush_flags check, empty flushes will complete here
if the device's request_queue doesn't advertise support for FLUSH or FUA
via blk_queue_flush().

Mike