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SubjectRe: [PATCH v2] block: fix mis-synchronisation in blkdev_issue_zeroout()
On 2011-03-11 15:31, Lukas Czerner wrote:
> On Fri, 11 Mar 2011, Jeff Moyer wrote:
>
>> Lukas Czerner <lczerner@redhat.com> writes:
>>
>>> 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>
>>> CC: Jens Axboe <axboe@kernel.dk>
>>> CC: Dmitry Monakhov <dmonakhov@openvz.org>
>>> CC: Jeff Moyer <jmoyer@redhat.com>
>>> ---
>>> block/blk-lib.c | 19 +++++++------------
>>> 1 files changed, 7 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>>> index eec78be..bd3e8df 100644
>>> --- 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 *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);
>>
>> I think bb will always be set here, no real need to check.
>>
>> Anyway, I though I already added my:
>>
>> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
>>
>> to this. No?
>>
>> Cheers,
>> Jeff
>
> Yes, you did and I forgot to add it into the patch. Sorry about that.

No worries, I added it now.

--
Jens Axboe



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