Messages in this thread | | | Date | Mon, 20 Apr 2020 19:32:15 +0300 | From | Petko Manolov <> | Subject | Re: [RFC] WRITE_ONCE_INC() and friends |
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On 20-04-20 08:05:45, 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.
There's pros and cons, as usual. Yet another macro(s) versus sorter/more readable code. This is why i decided to spam the list (and Paul) - in search for another opinion.
> > > 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.
Which would be unfortunate.
I just wish there was C type declaration that would force the compiler to do what READ/WRITE_ONCE() does now, but i also know this is too naive... :)
Petko
> > > I merely proposed the _INC() variant for better readability. > > > > More like shorter code lines :-) > > That too! ;-) > > Thanx, Paul
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