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

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Date:   Mon, 16 Nov 2020 15:53:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     corbet@....net, keescook@...omium.org, gregkh@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/13] seqnum_ops: Introduce Sequence Number Ops

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.