linux-kernel - Re: [PATCH 01/41] rxrpc: Fix RTT determination to use PING ACKs as a source

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Message-ID: <3399756.1699637142@warthog.procyon.org.uk>
Date:   Fri, 10 Nov 2023 17:25:42 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jeffrey E Altman <jaltman@...istor.com>
Cc:     dhowells@...hat.com, Marc Dionne <marc.dionne@...istor.com>,
        linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/41] rxrpc: Fix RTT determination to use PING ACKs as a source

Jeffrey E Altman <jaltman@...istor.com> wrote:

> > I do ignore ack.serial == 0 for this purpose.
> 
> Zero has the special meaning - this ACK is not explicitly in response to a
> received packet.
> 
> However, as mentioned, the serial number counter wraps frequently and most
> RxRPC implementations
> do not transition from serial 0xffffffff -> 0x00000001 when wrapping.

I don't skip zero serial numbers either.  I'm not sure whether it would be
better to do so.

> Otherwise, acked_serial = 0x01 will be considered smaller than orig_serial =
> 0xfffffffe and the slot will not be marked available.

As you mentioned in your follow up email, after() deals with that by casting
to signed, subtracting and then examining the result.

David