Re: [PATCH v2 05/13] tools/xenstore: make some write limit functions static

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/03/2023 9:50 am, Juergen Gross wrote:
> +static void wrl_xfer_credit(wrl_creditt *debit,  wrl_creditt debit_floor,
> +                         wrl_creditt *credit, wrl_creditt credit_ceil)
> +     /*
> +      * Transfers zero or more credit from "debit" to "credit".
> +      * Transfers as much as possible while maintaining
> +      * debit >= debit_floor and credit <= credit_ceil.
> +      * (If that's violated already, does nothing.)
> +      *
> +      * Sufficient conditions to avoid overflow, either of:
> +      *  |every argument| <= 0x3fffffff
> +      *  |every argument| <= 1E9
> +      *  |every argument| <= WRL_CREDIT_MAX
> +      * (And this condition is preserved.)
> +      */
> +{
> +     wrl_creditt xfer = MIN( *debit      - debit_floor,
> +                             credit_ceil - *credit      );

MIN() evaluates its parameters multiple times.  I believe the only legal
way for the compiler to emit this code is to interleave double reads.

As with pretty much any C code, you want to read the pointers into
locals first, then operate on them, then write them out at the end.

~Andrew