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

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Date:   Tue, 10 Nov 2020 15:58:48 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     corbet@....net, keescook@...omium.org, gregkh@...uxfoundation.org,
        peterz@...radead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 01/13] seqnum_ops: Introduce Sequence Number Ops

On 11/10/20 2:03 PM, Matthew Wilcox wrote:
> 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.

Yes we have some instances where unsigned is being used. I considered
going to unsigned. Changing the API to unsigned has other ramifications
and cascading changes to current atomic_t usages that are up counters.

git grep -E '\((unsigned|unsigned int|u32)\).*\batomic.*(read)' | wc -l
53

A total of 53 out of 6080 atomic_read() usages force return type to
unsigned.

git grep -E '\((unsigned|unsigned int|u32)\).*\batomic.*(inc_return)' | 
wc -l
11

A total of 11 out of 620 atomic_inc_return() usages force return type
to unsigned.

Changing the API to unsigned has other ramifications and cascading
changes to current atomic_t usages that are up counters.

We could add unsigned to seqnum_ops though.

> 
>> +++ 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.
> 

Thanks for catching these. This code needed cleanup after the
rename from a looong names from previous version.

thanks,
-- Shuah