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

[Max Reitz]

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?

>  * 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.

Max

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