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SubjectRe: Device loses barrier support (was: Fixed patch for simple barriers.)


On Fri, 5 Dec 2008, Andi Kleen wrote:

> > * barrier support in md-raid1 deviates from the specification at
> > Documentation/block/barrier.txt. The specification says that requests
> > submitted after the barrier request hit the media after the barrier
> > request hits the media. The reality is that the barrier request can be
> > randomly aborted and the requests submitted after it hit the media before
> > the barrier request.
>
> Yes the spec should be probably updated.
>
> But also see Linus' rant from yesterday about code vs documentation.
> When in doubt the code wins.

The only one offender is "md". It is less overhead to change "md" to play
nice and be reliable than to double-submit requests in all the places that
needs write ordering.

> > * the filesystems developed hacks to work around this issue, the hacks
> > involve not submitting more requests after the barrier request,
>
> I suspect the reason the file systems did it this way is that
> it was a much simpler change than to rewrite the transaction
> manager for this.

It could be initial reason. But this unreliability also disallows any
improvement in filesystems. No one can write asynchronous transaction
manager because of that evil EOPNOTSUPP.

> > synchronously waiting for the barrier request and eventually retrying it.
> > These hacks suppress any performance advantage barriers could bring.
> >
> > * you submit a patch that makes barriers even more often deviate from the
> > specification and you argue that the patch is correct because filesystems
> > handle this deviation.
>
> Sorry what counts is the code behaviour, not the specification.

Better interface is that one that has less maintenance overhead. And I
don't see requiring the programmers of all IO code to double-submit
requests as less maintenance overhead.

> -Andi

Mikulas

---
If you want to make it easier to infer functionality from the code, apply
this patch :)

---
block/blk-core.c | 8 ++++++++
1 file changed, 8 insertions(+)
Index: linux-2.6.28-rc5-devel/block/blk-core.c
===================================================================
--- linux-2.6.28-rc5-devel.orig/block/blk-core.c 2008-12-05 02:54:25.000000000 +0100
+++ linux-2.6.28-rc5-devel/block/blk-core.c 2008-12-05 03:14:23.000000000 +0100
@@ -28,6 +28,7 @@
#include <linux/task_io_accounting_ops.h>
#include <linux/blktrace_api.h>
#include <linux/fault-inject.h>
+#include <linux/random.h>

#include "blk.h"

@@ -1528,6 +1529,13 @@ void submit_bio(int rw, struct bio *bio)

bio->bi_rw |= rw;

+ /* At least, make the true nature of write barriers obvious. */
+
+ if (bio_barrier(bio) && !(random32() % 42)) {
+ bio_endio(bio, -EOPNOTSUPP);
+ return;
+ }
+
/*
* If it's a regular read/write or a barrier with data attached,
* go through the normal accounting stuff before submission.

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