netdev - RE: [PATCH net-next] r8169: Add 64bit statistics

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 17 Nov 2011 11:13:12 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Junchang Wang" <junchangwang@...il.com>,
	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Stephen Hemminger" <stephen.hemminger@...tta.com>,
	<netdev@...r.kernel.org>, <romieu@...zoreil.com>,
	"nic swsd" <nic_swsd@...ltek.com>
Subject: RE: [PATCH net-next] r8169: Add 64bit statistics

The 64bit stats update sequence used to get valid
counts on 32bit systems (that can't do locked 64bit
memory access) seems to be:

>     u64_stats_update_begin(&sky2->tx_stats.syncp);
>     ++sky2->tx_stats.packets;
>     sky2->tx_stats.bytes += skb->len;
>     u64_stats_update_end(&sky2->tx_stats.syncp);

I'm not sure what the begin/end markers do, but
they need to hold off the readers during updates
and the writers during reads - this is probably
expensive on the update path.

A thought that might work is for the writer to
write the middle bits of the 64 bit walue to
another location, eg:
       count = sky2->tx_stats.bytes + skb->len;
       sky2->tx_stats.bytes = count;
       sky2->tx_stats.bytes_check = count >> 16;
The reader then loops until the two value are
consistent.

I think this doesn't even require a memory barrier
in the ISR since the order of the reads an writes
doesn't matter at all.

	David


--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html