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

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Date:   Tue, 10 Nov 2020 21:03:16 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     corbet@....net, keescook@...omium.org, gregkh@...uxfoundation.org,
        peterz@...radead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/13] seqnum_ops: Introduce Sequence Number Ops

On Tue, Nov 10, 2020 at 12:53:27PM -0700, Shuah Khan wrote:
> Sequence Numbers wrap around to INT_MIN when it overflows and should not

Why would sequence numbers be signed?  I know they're built on top of
atomic_t, which is signed, but conceptually a sequence number is unsigned.

> +++ b/Documentation/core-api/seqnum_ops.rst
> @@ -0,0 +1,117 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. include:: <isonum.txt>
> +
> +.. _seqnum_ops:
> +
> +==========================
> +Sequence Number Operations
> +==========================
> +
> +:Author: Shuah Khan
> +:Copyright: |copy| 2020, The Linux Foundation
> +:Copyright: |copy| 2020, Shuah Khan <skhan@...uxfoundation.org>
> +
> +There are a number of atomic_t usages in the kernel where atomic_t api
> +is used strictly for counting sequence numbers and other statistical
> +counters and not for managing object lifetime.

You start by describing why this was introduced.  I think rather, you
should start by describing what this is.  You can compare and contrast
it with atomic_t later.  Also, I don't think it's necessary to describe
its implementation in this document.  This document should explain to
someone why they want to use this.

> +Read interface
> +--------------
> +
> +Reads and returns the current value. ::
> +
> +        seqnum32_read() --> atomic_read()
> +        seqnum64_read() --> atomic64_read()
> +
> +Increment interface
> +-------------------
> +
> +Increments sequence number and doesn't return the new value. ::
> +
> +        seqnum32_inc() --> atomic_inc()
> +        seqnum64_inc() --> atomic64_inc()

That seems odd to me.  For many things, I want to know what the
sequence number was incremented to.  Obviously seqnum_inc(); followed
by seqnum_read(); is racy.

Do we really want to be explicit about seqnum32 being 32-bit?
I'd be inclined to have seqnum/seqnum64 instead of seqnum32/seqnum64.

> +static inline int seqnum32_read(const struct seqnum32 *seq)
> +{
> +	return atomic_read(&seq->seqnum);
> +}
> +
> +/*
> + * seqnum32_set() - set seqnum value
> + * @seq: struct seqnum32 pointer
> + * @val: new value to set
> + *
> + */
> +static inline void
> +seqnum32_set(struct seqnum32 *seq, int val)

You have some odd formatting like the above line split.

> +static inline void seqnum64_dec(
> +				struct seqnum64 *seq)

That one is particularly weird.