Re: [Qemu-devel] [PATCH v8 00/36] block: Image locking series

[Max Reitz]

On 26.10.2016 17:33, Kevin Wolf wrote:
> Am 26.10.2016 um 17:12 hat Max Reitz geschrieben:
>> On 26.10.2016 13:01, Fam Zheng wrote:
>>> On Tue, 10/25 16:57, Kevin Wolf wrote:
>>>> Am 25.10.2016 um 15:30 hat Max Reitz geschrieben:
>>>>> On 25.10.2016 10:24, Kevin Wolf wrote:
>>>>>> Am 24.10.2016 um 20:03 hat Max Reitz geschrieben:
>>>>>>> On 24.10.2016 12:11, Kevin Wolf wrote:
>>>>>>>
>>>>>>> [...]
>>>>>>>
>>>>>>>> Now, the big question is how to translate this into file locking. This
>>>>>>>> could become a little tricky. I had a few thoughts involving another
>>>>>>>> lock on byte 2, but none of them actually worked out so far, because
>>>>>>>> what we want is essentially a lock that can be shared by readers, that
>>>>>>>> can also be shared by writers, but not by readers and writers at the
>>>>>>>> same time.
>>>>>>>
>>>>>>> You can also share it between readers and writers, as long as everyone
>>>>>>> can cope with volatile data.
>>>>>>
>>>>>> Sorry, that was ambiguous. I meant a file-level lock rather than the
>>>>>> high-level one. If we had a lock that can be shared by one or the other,
>>>>>> but not both, then two locks would be enough to build what we really
>>>>>> want.
>>>>>>
>>>>>>> I agree that it's very similar to the proposed op blocker style, but I
>>>>>>> can't really come up with a meaningful translation either.
>>>>>>>
>>>>>>> Maybe something like this (?): All readers who do not want the file to
>>>>>>> be modified grab a shared lock on byte 1. All writers who can deal with
>>>>>>> volatile data grab a shared lock on byte 2. Exclusive writers grab an
>>>>>>> exclusive lock on byte 1 and 2. Readers who can cope with volatile data
>>>>>>> get no lock at all.
>>>>>>>
>>>>>>> When opening, the first and second group would always have to test
>>>>>>> whether there is a lock on the other byte, respectively. E.g. sharing
>>>>>>> writers would first grab an exclusive lock on byte 1, then the shared
>>>>>>> lock on byte 2 and then release the exclusive lock again.
>>>>>>>
>>>>>>> Would that work?
>>>>>>
>>>>>> I'm afraid it wouldn't. If you start the sharing writer first and then
>>>>>> the writer-blocking reader, the writer doesn't hold a lock on byte 1 any
>>>>>> more,
>>>>>
>>>>> But it holds a lock on byte 2.
>>>>>
>>>>>>       so the reader can start even though someone is writing to the
>>>>>> image.
>>>>>
>>>>> It can't because it would try to grab an exclusive lock on byte 2 before
>>>>> grabbing the shared lock on byte 1.
>>>>
>>>> Apparently I failed to understand the most important part of the
>>>> proposal. :-)
>>>>
>>>> So we have two locks. Both are only held for a longer time in shared
>>>> mode. Exclusive mode is only used for testing whether the lock is being
>>>> held and is immediately given up again.
>>>>
>>>> The meaning of holding a shared lock is:
>>>>
>>>>     byte 1: I can't allow other processes to write to the image
>>>>     byte 2: I am writing to the image
>>>>
>>>> The four cases that we have involve:
>>>>
>>>> * shared writer: Take shared lock on byte 2. Test whether byte 1 is
>>>>   locked using an exclusive lock, and fail if so.
>>>>
>>>> * exclusive writer: Take shared lock on byte 2. Test whether byte 1 is
>>>>   locked using an exclusive lock, and fail if so. Then take shared lock
>>>>   on byte 1. I suppose this is racy, but we can probably tolerate that.
>>>>
>>>> * reader that can tolerate writers: Don't do anything
>>>>
>>>> * reader that can't tolerate writers: Take shared lock on byte 1. Test
>>>>   whether byte 2 is locked, and fail if so.
>>>>
>>>> Seems to work if I got that right.
>>>
>>> Does this mean I should change ImageLockMode to:
>>>
>>>  * exclusive
>>>  * shared-write
>>>  * shared-read
>>
>> Hm, those don't sound quite right, since it sounds as if you could mix
>> shared-read and shared-write. But you shouldn't be able to open an image
>> in shared-read lock mode when someone has opened it in shared-write lock
>> mode already.
>>
>> It's difficult to come up with a clear but short name for shared-read
>> ("exclusive", "shared-write", and "nolock" sound good to me). Maybe
>> "non-volatile" or "constant"? Or maybe "shared-only-read" would be clear
>> enough?
> 
> As we concluded above, this is really a matrix a two bools rather than a
> single property. We need a new option for "I can't allow other processes
> to write to the image", but we don't need one for "I am writing to the
> image" because that's the read-only property that we already have.
> 
>>>  * nolock
>>>  * auto
>>>
>>> Where "auto" maps to exclusive for O_RDWR and shared-read for O_RDONLY?
>>
>> Yep, that would be the correct mapping. Maybe later we can introduce an
>> auto-shared mode that maps to shared-write or nolock, respectively.
> 
> No auto needed any more, the default is simply false (i.e. don't share
> with other writers).

Well, that was too easy. :-)

Max

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