netdev - Re: bug in bonding driver

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Message-ID: <4BAA6C49.3040404@nortel.com>
Date:	Wed, 24 Mar 2010 13:47:21 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Jay Vosburgh <fubar@...ibm.com>
CC:	netdev@...r.kernel.org, bonding-devel@...ts.sourceforge.net
Subject: Re: bug in bonding driver

On 03/24/2010 01:13 PM, Jay Vosburgh wrote:
> Chris Friesen <cfriesen@...tel.com> wrote:

>> The catch here is that slave->jiffies may not ever get updated after
>> being set initially, and on long-running systems jiffies will overflow.
>> That could cause this check to be true for a substantial amount of time
>> rather than for just a short period.

> 	Some quick fooling around suggests that if, for example,
> slave->jiffies is near the top of the range (ULONG_MAX - a few hundred),
> when jiffies gets up to around ULONG_MAX / 2 time_after_eq will flip
> from "after" to "before."
> 
> 	I don't think this is a particularly farfetched example, since
> jiffies is intentionally started near the top of the range, so
> slave->jiffies is likely to be high in the range after bonding is
> configured at boot.

Agreed.  If I understand it right the result of time_after_eq is only
valid if the result of the subtraction is less than LONG_MAX.

>> One way to fix it would be a boolean which tracks whether or not we've
>> gone past the time, and if we have then we don't bother actually
>> checking the time anymore.
> 
> 	It might be clearer to make the slave->jiffies some kind of
> countdown instead, perhaps reusing the slave->delay used for
> updelay/downdelay and eliminating slave->jiffies entirely.

Yes, that seems reasonable as well.

Chris
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