netdev - Re: [PATCH] tcp_probe: avoid modulus operation and wrap fix

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Message-Id: <20100125.154814.56026460.davem@davemloft.net>
Date:	Mon, 25 Jan 2010 15:48:14 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp_probe: avoid modulus operation and wrap fix

From: Stephen Hemminger <shemminger@...tta.com>
Date: Mon, 25 Jan 2010 08:30:06 -0800

> On Sun, 24 Jan 2010 22:41:22 -0800 (PST)
> David Miller <davem@...emloft.net> wrote:
> 
>> From: Stephen Hemminger <shemminger@...tta.com>
>> Date: Sun, 24 Jan 2010 21:50:01 -0800
>> 
>> > @@ -75,12 +75,12 @@ static struct {
>> >  
>> >  static inline int tcp_probe_used(void)
>> >  {
>> > -	return (tcp_probe.head - tcp_probe.tail) % bufsize;
>> > +	return (tcp_probe.head - tcp_probe.tail) & (bufsize - 1);
>> >  }
>> >  
>> >  static inline int tcp_probe_avail(void)
>> >  {
>> > -	return bufsize - tcp_probe_used();
>> > +	return bufsize - tcp_probe_used() - 1;
>> >  }
>> >  
>> >  /*
>> 
>> Hmmm...  When the ring is full head==tail, which means
>> tcp_probe_used() returns 0.  Which would now make tcp_probe_avail()
>> return "bufsize - 0 - 1".
>> 
>> Is that right?
> 
> Yes. in this ring; empty is head == tail, and full needs to
> be tail == head - 1.

Ok, that makes sense, thanks for explaining.

Applied, thanks Stephen.
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