Messages in this thread | | | Date | Tue, 21 Apr 2020 06:19:19 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC] WRITE_ONCE_INC() and friends |
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On Tue, Apr 21, 2020 at 11:33:57AM +0200, Marco Elver wrote: > On Tue, 21 Apr 2020 at 01:12, Paul E. McKenney <paulmck@kernel.org> wrote: > > > > On Tue, Apr 21, 2020 at 12:57:15AM +0200, Marco Elver wrote: > > > On Mon, 20 Apr 2020, Paul E. McKenney wrote: > > > > > > > On Sun, Apr 19, 2020 at 09:37:10PM +0000, David Laight wrote: > > > > > From: Petko Manolov > > > > > > Sent: 19 April 2020 19:30 > > > > > > > > > > > > On 20-04-19 18:02:50, David Laight wrote: > > > > > > > From: Petko Manolov > > > > > > > > Sent: 19 April 2020 10:45 > > > > > > > > Recently I started reading up on KCSAN and at some point I ran into stuff like: > > > > > > > > > > > > > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1); > > > > > > > > WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); > > > > > > > > > > > > > > If all the accesses use READ/WRITE_ONCE() why not just mark the structure > > > > > > > field 'volatile'? > > > > > > > > > > > > This is a bit heavy. I guess you've read this one: > > > > > > > > > > > > https://lwn.net/Articles/233479/ > > > > > > > > > > I remember reading something similar before. > > > > > I also remember a very old gcc (2.95?) that did a readback > > > > > after every volatile write on sparc (to flush the store buffer). > > > > > That broke everything. > > > > > > > > > > I suspect there is a lot more code that is attempting to be lockless > > > > > these days. > > > > > Ring buffers (one writer and one reader) are a typical example where > > > > > you don't need locks but do need to use a consistent value. > > > > > > > > > > Now you may also need ordering between accesses - which I think needs > > > > > more than volatile. > > > > > > > > In Petko's patch, all needed ordering is supplied by the fact that it > > > > is the same variable being read and written. But yes, in many other > > > > cases, more ordering is required. > > > > > > > > > > And no, i am not sure all accesses are through READ/WRITE_ONCE(). If, for > > > > > > example, all others are from withing spin_lock/unlock pairs then we _may_ not > > > > > > need READ/WRITE_ONCE(). > > > > > > > > > > The cost of volatile accesses is probably minimal unless the > > > > > code is written assuming the compiler will only access things once. > > > > > > > > And there are variables marked as volatile, for example, jiffies. > > > > > > > > But one downside of declaring variables volatile is that it can prevent > > > > KCSAN from spotting violations of the concurrency design for those > > > > variables. > > > > > > Note that, KCSAN currently treats volatiles not as special, except a > > > list of some known global volatiles (like jiffies). This means, that > > > KCSAN will tell us about data races involving unmarked volatiles (unless > > > they're in the list). > > > > > > As far as I can tell, this is what we want. At least according to LKMM. > > > > > > If, for whatever reason, volatiles should be treated differently, we'll > > > have to modify the compilers to emit different instrumentation for the > > > kernel. > > > > I stand corrected, then, thank you! > > > > In the current arrangement, declaring a variable volatile will cause > > KCSAN to generate lots of false positives. > > > > I don't currently have a strong feeling on changing the current situation > > with respect to volatile variables. Is there a strong reason to change? > > The general view of the community, as you say, has been that you don't use > > the volatile keyword outside of exceptions such as jiffies, atomic_read(), > > atomic_set(), READ_ONCE(), WRITE_ONCE() and perhaps a few others. > > > > Thoughts? > > I certainly agree, and also want to point out that checkpatch.pl > complains about volatile. We know using volatile has problems. KCSAN > is (along with checkpatch.pl) another tool that can warn us about such > problems (warning in case there is real concurrency). Another thing to > point out is that volatile is not portable, in case > READ_ONCE()/WRITE_ONCE()'s smp_load_barrier_depends() is not a noop. > So from what I see, there are strong reasons against changing the > situation for volatiles and KCSAN.
All good points, thank you!
Thanx, Paul
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