Re: [PATCH v2 01/13] seqnum_ops: Introduce Sequence Number Ops

[Peter Zijlstra]

On Fri, Nov 13, 2020 at 10:46:03AM -0700, Shuah Khan wrote:

> +Increment interface
> +-------------------
> +
> +Increments sequence number and returns the new value. ::
> +
> +        seqnum32_inc_return() --> (u32) atomic_inc_return(seqnum)
> +        seqnum64_inc_return() --> (u64) atomic64_inc_return(seqnum)

Did you think about the ordering?

> +Fetch interface
> +---------------
> +
> +Fetched and returns current sequence number value. ::
> +
> +        seqnum32_fetch() --> (u32) atomic_add_return(0, seqnum)
> +        seqnum64_fetch() --> (u64) atomic64_add_return(0, seqnum)

That's horrible. Please explain how that is not broken garbage.

Per the fact that it is atomic, nothing prevents the counter being
incremented concurrently. There is no such thing as a 'current' sequence
number.