linux-kernel - Block devices and barriers

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Message-ID: <Pine.LNX.4.62.0705211434150.8776@pademelon.sonytel.be>
Date:	Mon, 21 May 2007 15:16:33 +0200 (CEST)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	Linux Kernel Development <linux-kernel@...r.kernel.org>
Subject: Block devices and barriers

	Hi,

I'm adding cache flush support to the PS3 disk driver and have a few questions
related to barriers.

In my driver, I do:

    blk_queue_issue_flush_fn(queue, ps3disk_issue_flush);
    blk_queue_ordered(queue, QUEUE_ORDERED_DRAIN_FLUSH, ps3disk_prepare_flush);

but I can't find a way to actually trigger the calling of ps3disk_issue_flush()
and ps3disk_prepare_flush().

 1. My prepare_flush_fn() routine should be called from queue_flush(), which is
    in turn called by start_ordered().
    start_ordered() is called by blk_do_ordered(), but only if there's no
    barrier (REQ_HARDBARRIER is not set).
    Apart from drivers/block/pktcdvd.c and init_request_from_bio()
    (BIO_RW_BARRIER is set by drivers/md/md.c only?), the only other way
    REQ_HARDBARRIER can be set is in queue_flush(), which is not possible.

  2. My issue_flush_fn() should be called from blkdev_issue_flush() (ignoring
     drivers/md). But blkdev_issue_flush() is called by ReiserFS and XFS only.

I read the Block Device chapter in Linux Device Drivers 3rd edition, which says
to check blk_barrier_rq(), but the barrier part seems to be obsolete, as we now
have the prepare_flush_fn parameter of blk_queue_ordered().

Am I missing something? Should I care about barriers?

Thanks!

BTW, Documentation/block/barrier.txt seems to be out-of-date. Patch to update
it is below.

---

Documentation/block/barrier.txt is not in sync with the actual code:
  - blk_queue_ordered() no longer has a gfp_mask parameter
  - blk_queue_ordered_locked() no longer exists
  - sd_prepare_flush() looks slightly different

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>

--- a/Documentation/block/barrier.txt
+++ b/Documentation/block/barrier.txt
@@ -82,23 +82,12 @@ including draining and flushing.
 typedef void (prepare_flush_fn)(request_queue_t *q, struct request *rq);
 
 int blk_queue_ordered(request_queue_t *q, unsigned ordered,
-		      prepare_flush_fn *prepare_flush_fn,
-		      unsigned gfp_mask);
-
-int blk_queue_ordered_locked(request_queue_t *q, unsigned ordered,
-			     prepare_flush_fn *prepare_flush_fn,
-			     unsigned gfp_mask);
-
-The only difference between the two functions is whether or not the
-caller is holding q->queue_lock on entry.  The latter expects the
-caller is holding the lock.
+		      prepare_flush_fn *prepare_flush_fn);
 
 @q			: the queue in question
 @ordered		: the ordered mode the driver/device supports
 @prepare_flush_fn	: this function should prepare @rq such that it
 			  flushes cache to physical medium when executed
-@..._mask		: gfp_mask used when allocating data structures
-			  for ordered processing
 
 For example, SCSI disk driver's prepare_flush_fn looks like the
 following.
@@ -106,9 +95,10 @@ following.
 static void sd_prepare_flush(request_queue_t *q, struct request *rq)
 {
 	memset(rq->cmd, 0, sizeof(rq->cmd));
-	rq->flags |= REQ_BLOCK_PC;
+	rq->cmd_type = REQ_TYPE_BLOCK_PC;
 	rq->timeout = SD_TIMEOUT;
 	rq->cmd[0] = SYNCHRONIZE_CACHE;
+	rq->cmd_len = 10;
 }
 
 The following seven ordered modes are supported.  The following table

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- Sony Network and Software Technology Center Europe (NSCE)
Geert.Uytterhoeven@...ycom.com ------- The Corporate Village, Da Vincilaan 7-D1
Voice +32-2-7008453 Fax +32-2-7008622 ---------------- B-1935 Zaventem, Belgium
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