linux-kernel - Re: [PATCH] rbd: revalidate

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Date:	Thu, 11 Apr 2013 08:27:36 -0500
From:	Alex Elder <elder@...tank.com>
To:	Laurent Barbe <laurent@...eris.com>
CC:	ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rbd: revalidate_disk upon rbd resize

On 04/11/2013 03:47 AM, Laurent Barbe wrote:
> Thanks Alex,
> 
> What do you mean about "lock inversion", deadlock ?
> Is it the mutex_lock in block_dev.c:revalidate_disk() and the mutex
> in rbd_dev_refresh ?

Yes.

When we refresh an rbd device we get the rbd device header
semaphore, and with this patch we then acquire the bdev's
mutex.

Meanhwhile, via blkdev_close() __blkdev_put() acquires the
bdev->bd_mutex, and eventually we get down to creating an
image object request.  If it's a write, we need to get the
snapshot context, and to do that we get the rbd device
header mutex.

So we're acquiring the locks in two different orders, and
that's what I meant by "lock inversion," and yes, that
can lead to deadlock.

There may be a simple fix--like holding off calling
revalidate_disk() until after we release the lock,
most likely in rbd_dev_refresh().  But I just haven't
really thought it through yet.

					-Alex

> 
> 2013/4/11 Alex Elder <elder@...tank.com>
> 
>> On 04/10/2013 02:30 PM, Alex Elder wrote:
>>> On 04/10/2013 11:06 AM, Laurent Barbe wrote:
>>>> If rbd disk is open and rbd resize is done, new size is not visible by
>>>> filesystem.
>>>> Like is done in virtio-blk and dm driver, revalidate_disk() permits to
>>>> update the bd_inode size.
>>>
>>> Looks good to me.  I'll take this in via the ceph-client tree.
>>> Thanks a lot.
>>
>> Unfortunately this leads to a lock inversion.  I'm going to
>> think about how to go about resolving it, so I won't be
>> committing it just yet.
>>
>>                                         -Alex
>>
>>>
>>> Reviewed-by: Alex Elder <elder@...tank.com>
>>>
>>>> Signed-off-by: Laurent Barbe <laurent@...eris.com>
>>>> ---
>>>>  drivers/block/rbd.c |    1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>> index f556f8a..1963025 100644
>>>> --- a/drivers/block/rbd.c
>>>> +++ b/drivers/block/rbd.c
>>>> @@ -2293,6 +2293,7 @@ static void rbd_update_mapping_size(struct
>> rbd_device *rbd_dev)
>>>>      dout("setting size to %llu sectors", (unsigned long long) size);
>>>>      rbd_dev->mapping.size = (u64) size;
>>>>      set_capacity(rbd_dev->disk, size);
>>>> +    revalidate_disk(rbd_dev->disk);
>>>>  }
>>>>
>>>>  /*
>>>>
>>>
>>
>>
> 

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