Re: [Xen-devel] [PATCH v5 3/4] xenstore: rework of transaction handling

[Juergen Gross <jgross@xxxxxxxx>]

On 31/03/17 15:01, Wei Liu wrote:
> On Fri, Mar 31, 2017 at 02:11:46PM +0200, Juergen Gross wrote:
>> On 31/03/17 14:03, Wei Liu wrote:
>>> Another stupid question below.
>>>
>>> On Fri, Mar 31, 2017 at 01:29:19PM +0200, Juergen Gross wrote:
>>>>  
>>>> -struct changed_node
>>>> +/*
>>>> + * Some notes regarding detection and handling of transaction conflicts:
>>>> + *
>>>> + * Basic source of reference is the 'generation' count. Each writing 
>>>> access
>>>> + * (either normal write or in a transaction) to the tdb data base will set
>>>> + * the node specific generation count to the global generation count.
>>>> + * For being able to identify a transaction the transaction specific 
>>>> generation
>>>> + * count is initialized with the global generation count when starting the
>>>> + * transaction.
>>>> + * Each time the global generation count is copied to either a node or a
>>>> + * transaction it is incremented. This ensures all nodes and/or 
>>>> transactions
>>>> + * are having a unique generation count.
>>>> + *
>>>> + * Transaction conflicts are detected by checking the generation count of 
>>>> all
>>>> + * nodes read in the transaction to match with the generation count in the
>>>> + * global data base at the end of the transaction. Nodes which have been
>>>> + * modified in the transaction don't have to be checked to match even if 
>>>> they
>>>> + * have been read, as the modified node will be globally visible after the
>>>> + * succeeded transaction possibly overwriting another modification which 
>>>> may
>>>> + * have occurred concurrent to the transaction.
>>>> + *
>>>> + * Examples:
>>>> + * ---------
>>>> + * The following notation is used:
>>>> + * I:      initial state
>>>> + * G       global generation count
>>>> + * g(X)    generation count of node X
>>>> + * G(1)    generation count of transaction 1
>>>> + * g(1:Y)  saved generation count of node Y in transaction 1
>>>
>>> Assuming this is recorded in accessed_node, can you point me to where
>>> about in the code this is implemented? I looked at access_node but that
>>> only records generation for read.
>>
>> It is in the node itself stored in the data base.
>>
> 
> Right, then ...
> 
>>>> + * TA1:    operation in transaction 1
>>>> + * X=1:X   replace global node X with transaction 1 specific value of X
>>>> + *
>>>> + * 1. Simple transaction doing: read node A, write node B
>>>> + *    I: g(A) = 1, g(B) = 2, G = 3
>>>> + *    Start transaction 1: G(1) = 3, G = 4
>>>> + *    TA1: read node A:    g(1:A) = 1
>>>> + *    TA1: write node B:   g(1:B) = 4, G = 5
>>>
> 
> I suggest updating places like g(1:B) = 4 to g(B) = 4 to match the code.

No, as during the transaction only the node with the transaction
prepended is being stored in the data base. It isn't visible outside
the transaction until the transaction has been committed.

So the notation isn't reflecting where the generation count is stored,
but only that it is specific to the transaction/node pair.


Juergen

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