Messages in this thread | | | Date | Wed, 6 Jun 2012 19:17:15 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH] virtio-net: fix a race on 32bit arches |
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On Wed, Jun 06, 2012 at 05:19:04PM +0200, Eric Dumazet wrote: > On Wed, 2012-06-06 at 17:49 +0300, Michael S. Tsirkin wrote: > > On Wed, Jun 06, 2012 at 03:10:10PM +0200, Eric Dumazet wrote: > > > On Wed, 2012-06-06 at 14:13 +0300, Michael S. Tsirkin wrote: > > > > > > > We currently do all stats either on napi callback or from > > > > start_xmit callback. > > > > This makes them safe, yes? > > > > > > Hmm, then _bh() variant is needed in virtnet_stats(), as explained in > > > include/linux/u64_stats_sync.h section 6) > > > > > > * 6) If counter might be written by an interrupt, readers should block interrupts. > > > * (On UP, there is no seqcount_t protection, a reader allowing interrupts could > > > * read partial values) > > > > > > Yes, its tricky... > > > > Sounds good, but I have a question: this realies on counters > > being atomic on 64 bit. > > Would not it be better to always use a seqlock even on 64 bit? > > This way counters would actually be correct and in sync. > > As it is if we want e.g. average packet size, > > we can not rely e.g. on it being bytes/packets. > > When this stuff was discussed, we chose to have a nop on 64bits. > > Your point has little to do with 64bit stats, it was already like that > with 'long int' counters.
Yes, of course.
> Consider average driver doing : > > dev->stats.rx_bytes += skb->len; > dev->stats.rx_packets++; > > A concurrent reader can read an updated rx_bytes and a 'previous' > rx_packets one. > > 'fixing' this requires a lot of work and memory barriers (in all > drivers), for a very litle gain (at most one packet error) > u64_stats_sync was really meant to be 0-cost on 64bit arches. > >
I understand, and not arguing about that.
But why do you say at most 1 packet?
Consider get_stats doing: u64_stats_update_begin(&stats->syncp); stats->tx_bytes += skb->len;
on 64 bit at this point tx_packets might get incremented any number of times, no?
stats->tx_packets++; u64_stats_update_end(&stats->syncp);
now tx_bytes and tx_packets are out of sync by more than 1.
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