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block: fix mis-synchronisation in blkdev_issue_zeroout() 

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>
This commit is contained in:
Lukas Czerner 2011-03-11 10:23:53 +01:00 committed by Jens Axboe
parent 9179746652
commit 0aeea18964
1 changed files with 7 additions and 12 deletions
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19
block/blk-lib.c
View File

@ -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 *bio, int err)
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_device *bdev, sector_t sector,
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))