netdev - RE: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B7396@saturn3.aculab.com>
Date:	Fri, 18 Oct 2013 09:40:33 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Jan Beulich" <JBeulich@...e.com>, "annie li" <annie.li@...cle.com>
Cc:	<david.vrabel@...rix.com>, <ian.campbell@...rix.com>,
	<wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	"jianhai luan" <jianhai.luan@...cle.com>, <netdev@...r.kernel.org>
Subject: RE: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after_eq().

> > My understanding is this patch does not simply double the span, it is
> > just stricter than the original one. Please check my previous comments,
> > I paste it here.
> 
> No, the code (on a 32-bit arch) just _can't_ handle jiffies differences
> beyond 2^32, no matter how cleverly you use the respective macros.
> All arithmetic there is done modulo 2^32.

I haven't followed this discussion very closely but it might be possible
to arrange that the 'incorrect lack of credit' only occurs for a few
seconds every time 'jiffies' wraps - instead of half of the time.
Then you'd have to be extremely unlucky to hit the timing window.

	David



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