[PATCH v3] md: protect against crash upon fsync on ro array

[Sebastian Riemer <sebastian.riemer@profitbricks.com>]

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On 28.01.2013 11:32, Sebastian Riemer wrote:
> O.K., then I hope Neil applies the attached patch. I've changed the
> return value to success.
> 
> This is also something for linux-stable and should apply to many kernel
> versions without an issue.
> 

I've tried to race with continuous fsyncs against continuous "mdadm -o
/dev/md0; mdadm -w /dev/md0;" in parallel but couldn't break it.
Therefore, no additional locking is required and this part can be put
directly before the RCU locking stuff and the suspended handling.

I've attached version 3 of the patch.

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From fe0357344877c9b9cc623fd582a4e0670e448317 Mon Sep 17 00:00:00 2001
From: Sebastian Riemer <sebastian.riemer@profitbricks.com>
Date: Fri, 25 Jan 2013 12:46:59 +0100
Subject: [PATCH v3] md: protect against crash upon fsync on ro array

If an fsync occurrs on a read-only array, we need to send a
completion for the IO and may not increment the active IO count.
Otherwise, we hit a bug trace and can't stop the MD array anymore.

By advice of Christoph Hellwig we silently return success.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: NeilBrown <neilb@suse.de>
Signed-off-by: Sebastian Riemer <sebastian.riemer@profitbricks.com>
Reported-by: Ben Hutchings <ben@decadent.org.uk>

---
 drivers/md/md.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 3db3d1b..6ba20f7 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -307,6 +307,10 @@ static void md_make_request(struct request_queue *q, struct bio *bio)
 		bio_io_error(bio);
 		return;
 	}
+	if (mddev->ro == 1 && unlikely(rw == WRITE)) {
+		bio_endio(bio, 0);
+		return;
+	}
 	smp_rmb(); /* Ensure implications of  'active' are visible */
 	rcu_read_lock();
 	if (mddev->suspended) {