Messages in this thread | | | Date | Thu, 12 May 2022 16:10:31 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH net] tcp: Add READ_ONCE() to read tcp_orphan_count |
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On Thu, May 12, 2022 at 02:31:48PM -0700, Eric Dumazet wrote: > On Thu, May 12, 2022 at 2:18 PM Marco Elver <elver@google.com> wrote: > > > > > I guess the question is, is it the norm that per_cpu() retrieves data > > that can legally be modified concurrently, or not. If not, and in most > > cases it's a bug, the annotations should be here. > > > > Paul, was there any guidance/documentation on this, but I fail to find > > it right now? (access-marking.txt doesn't say much about per-CPU > > data.) > > Normally, whenever we add a READ_ONCE(), we are supposed to add a comment.
I am starting to think that comments are even more necessary for unmarked accesses to shared variables, with the comments setting out why the compiler cannot mess things up. ;-)
> We could make an exception for per_cpu_once(), because the comment > would be centralized > at per_cpu_once() definition.
This makes a lot of sense to me.
> We will be stuck with READ_ONCE() in places we are using > per_cpu_ptr(), for example > in dev_fetch_sw_netstats()
If this is strictly statistics, data_race() is another possibility. But it does not constrain the compiler at all.
Thanx, Paul
> diff --git a/net/core/dev.c b/net/core/dev.c > index 1461c2d9dec8099a9a2d43a704b4c6cb0375f480..b66470291d7b7e6c33161093d71e40587f9ed838 > 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -10381,10 +10381,13 @@ void dev_fetch_sw_netstats(struct > rtnl_link_stats64 *s, > stats = per_cpu_ptr(netstats, cpu); > do { > start = u64_stats_fetch_begin_irq(&stats->syncp); > - tmp.rx_packets = stats->rx_packets; > - tmp.rx_bytes = stats->rx_bytes; > - tmp.tx_packets = stats->tx_packets; > - tmp.tx_bytes = stats->tx_bytes; > + /* These values can change under us. > + * READ_ONCE() pair with too many write sides... > + */ > + tmp.rx_packets = READ_ONCE(stats->rx_packets); > + tmp.rx_bytes = READ_ONCE(stats->rx_bytes); > + tmp.tx_packets = READ_ONCE(stats->tx_packets); > + tmp.tx_bytes = READ_ONCE(stats->tx_bytes); > } while (u64_stats_fetch_retry_irq(&stats->syncp, start)); > > s->rx_packets += tmp.rx_packets;
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