netdev - RE: [PATCH BUGFIX 2/6] pkt_sched: fix the update of eligible-group sets

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B7178@saturn3.aculab.com>
Date:	Wed, 6 Mar 2013 10:05:58 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Paolo valente" <paolo.valente@...more.it>,
	"Jamal Hadi Salim" <jhs@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>, <shemminger@...tta.com>
Cc:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<fchecconi@...il.com>, <rizzo@....unipi.it>
Subject: RE: [PATCH BUGFIX 2/6] pkt_sched: fix the update of eligible-group sets

> Between two invocations of make_eligible, the system virtual time may
> happen to grow enough that, in its binary representation, a bit with
> higher order than 31 flips. This happens especially with
> TSO/GSO. Before this fix, the mask used in make_eligible was computed
> as (1UL<<index_of_last_flipped_bit)-1, whose value is well defined on
> a 64-bit architecture, because index_of_flipped_bit <= 63, but is in
> general undefined on a 32-bit architecture if index_of_flipped_bit > 31.
> The fix just replaces 1UL with 1ULL.
...
> -		unsigned long mask = (1UL << fls(vslot ^ old_vslot)) - 1;
> +		unsigned long mask = (1ULL << fls(vslot ^ old_vslot)) - 1;

I'm not sure you really want to be doing 64bit shifts on 32 bit
systems just to generate ~0ul.
probably worth a conditional.

	David


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