linux-kernel - Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy

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Date:	Thu, 13 Sep 2012 09:12:25 +1000
From:	Joseph Glanville <joseph.glanville@...onvm.com.au>
To:	Tejun Heo <tj@...nel.org>
Cc:	NeilBrown <neilb@...e.de>,
	Kent Overstreet <koverstreet@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Philipp Reisner <philipp.reisner@...bit.com>,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Vivek Goyal <vgoyal@...hat.com>, drbd-dev@...ts.linbit.com
Subject: Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy

On 13 September 2012 04:58, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Tue, Sep 11, 2012 at 10:25:01AM +0200, Lars Ellenberg wrote:
>> Or do we have a better place to document write ordering requirements?
>>
>> "To enforce write-after-write dependencies, you *have* to drain the
>> queue (do we have a generic interface available for that?),
>> or at least wait for the completion of all the requests you
>> (potentially) depend upon, before even submitting the dependent request.
>>
>> Additionally, to avoid integrity issues due to volatile caches,
>> you need to use FLUSH/FUA as appropriate."
>
> I think it would be best to note the lack of any ordering guarantee in
> the comment of bio_submit()?  Any one interested in submitting a
> patch?
>
> Thanks.
>
> --
> tejun
> --
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Something along the lines of this?

diff --git a/block/blk-core.c b/block/blk-core.c
index 4b4dbdf..68b5671 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1809,6 +1809,9 @@ EXPORT_SYMBOL(generic_make_request);
  * uses that function to do most of the work. Both are fairly rough
  * interfaces; @bio must be presetup and ready for I/O.
  *
+ * Ordering of requests is not guaranteed, callers should drain the queue
+ * and issue a flush before submission of any dependent bios.
+ *
  */
 void submit_bio(int rw, struct bio *bio)
 {

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