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

[Juergen Gross <jgross@xxxxxxxx>]

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.

>> + * 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
> 
> Specifically, here you mention write type also records generation count.

In the end the generation count of a node modified by a transaction
isn't important for the transaction.


Juergen


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