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

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Date:   Tue, 17 Nov 2020 09:15:34 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     corbet@....net, keescook@...omium.org, gregkh@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org
Subject: Re: [PATCH v2 01/13] seqnum_ops: Introduce Sequence Number Ops

On 11/16/20 7:53 AM, Peter Zijlstra wrote:
> 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?
> 

Looking at atomic_t.txt _inc_return() can be fully ordered without
loosing or making the intermediate state visible. This is good for
this sequence number use-case. Is there something I am overlooking?

>> +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.
> 

Correct. Some usages of this _fecth() in this patch series are for
printing sequence numbers in debug message and others are stats.

I will review the patches in this series and drop the ones that use
read/fetch - the reason being sequence numbers are strictly up counters
and don't need read/fetch.

thanks,
-- Shuah