Messages in this thread | | | Date | Tue, 17 May 2022 13:28:28 -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 04:43:20PM -0700, Eric Dumazet wrote: > On Thu, May 12, 2022 at 4:10 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > 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. > > Statistics are supposed to be monotonically increasing ;) > > Some SNMP agents would be very confused if they could observe 'garbage' there. > > I sense that we are going to add thousands of READ_ONCE() soon :/
Indeed, adding READ_ONCE() instances can be annoying. Then again, it can also be annoying to have to debug the problems that sometimes arise from omitting them where they are needed.
Thanx, Paul
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