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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

I will continue my review.

Wei.

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