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Subject[16/87] block: fix mis-synchronisation in blkdev_issue_zeroout()
2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Lukas Czerner <lczerner@redhat.com>

commit 0aeea18964173715a1037034ef6838198f319319 upstream.

BZ29402
https://bugzilla.kernel.org/show_bug.cgi?id=29402

We can hit serious mis-synchronization in bio completion path of
blkdev_issue_zeroout() leading to a panic.

The problem is that when we are going to wait_for_completion() in
blkdev_issue_zeroout() we check if the bb.done equals issued (number of
submitted bios). If it does, we can skip the wait_for_completition()
and just out of the function since there is nothing to wait for.
However, there is a ordering problem because bio_batch_end_io() is
calling atomic_inc(&bb->done) before complete(), hence it might seem to
blkdev_issue_zeroout() that all bios has been completed and exit. At
this point when bio_batch_end_io() is going to call complete(bb->wait),
bb and wait does not longer exist since it was allocated on stack in
blkdev_issue_zeroout() ==> panic!

(thread 1) (thread 2)
bio_batch_end_io() blkdev_issue_zeroout()
if(bb) { ...
if (bb->end_io) ...
bb->end_io(bio, err); ...
atomic_inc(&bb->done); ...
... while (issued != atomic_read(&bb.done))
... (let issued == bb.done)
... (do the rest of the function)
... return ret;
complete(bb->wait);
^^^^^^^^
panic
We can fix this easily by simplifying bio_batch and completion counting.

Also remove bio_end_io_t *end_io since it is not used.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Eric Whitney <eric.whitney@hp.com>
Tested-by: Eric Whitney <eric.whitney@hp.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
CC: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
block/blk-lib.c | 19 +++++++------------
1 file changed, 7 insertions(+), 12 deletions(-)
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -109,7 +109,6 @@ struct bio_batch
atomic_t done;
unsigned long flags;
struct completion *wait;
- bio_end_io_t *end_io;
};

static void bio_batch_end_io(struct bio *bio, int err)
@@ -122,12 +121,9 @@ static void bio_batch_end_io(struct bio
else
clear_bit(BIO_UPTODATE, &bb->flags);
}
- if (bb) {
- if (bb->end_io)
- bb->end_io(bio, err);
- atomic_inc(&bb->done);
- complete(bb->wait);
- }
+ if (bb)
+ if (atomic_dec_and_test(&bb->done))
+ complete(bb->wait);
bio_put(bio);
}

@@ -150,13 +146,12 @@ int blkdev_issue_zeroout(struct block_de
int ret;
struct bio *bio;
struct bio_batch bb;
- unsigned int sz, issued = 0;
+ unsigned int sz;
DECLARE_COMPLETION_ONSTACK(wait);

- atomic_set(&bb.done, 0);
+ atomic_set(&bb.done, 1);
bb.flags = 1 << BIO_UPTODATE;
bb.wait = &wait;
- bb.end_io = NULL;

submit:
ret = 0;
@@ -185,12 +180,12 @@ submit:
break;
}
ret = 0;
- issued++;
+ atomic_inc(&bb.done);
submit_bio(WRITE, bio);
}

/* Wait for bios in-flight */
- while (issued != atomic_read(&bb.done))
+ if (!atomic_dec_and_test(&bb.done))
wait_for_completion(&wait);

if (!test_bit(BIO_UPTODATE, &bb.flags))



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