On Thu, Mar 07, 2024 at 12:44:39AM -0500, Joel Fernandes wrote:
>
>
> On 3/6/2024 10:37 PM, Paul E. McKenney wrote:
> > On Wed, Mar 06, 2024 at 10:06:21PM -0500, Mathieu Desnoyers wrote:
> >> On 2024-03-06 21:43, Linus Torvalds wrote:
> >> [...]
> >>>
> >>> Honestly, this all makes me think that we'd be *much* better off
> >>> showing the real "handoff" with smp_store_release() and
> >>> smp_load_acquire().
> >>
> >> We've done something similar in liburcu (userspace code) to allow
> >> Thread Sanitizer to understand the happens-before relationships
> >> within the RCU implementations and lock-free data structures.
> >>
> >> Moving to load-acquire/store-release (C11 model in our case)
> >> allowed us to provide enough happens-before relationship for
> >> Thread Sanitizer to understand what is happening under the
> >> hood in liburcu and perform relevant race detection of user
> >> code.
> >
> > Good point!
> >
> > In the kernel, though, KCSAN understands the Linux-kernel memory model,
> > and so we don't get that sort of false positive.
> >
> >> As far as the WRITE_ONCE(x, READ_ONCE(x) + 1) pattern
> >> is concerned, the only valid use-case I can think of is
> >> split counters or RCU implementations where there is a
> >> single updater doing the increment, and one or more
> >> concurrent reader threads that need to snapshot a
> >> consistent value with READ_ONCE().
> >
> > It is wrong here. OK, not wrong from a functional viewpoint, but it
> > is at best very confusing. I am applying patches to fix this. I will
> > push out a new "dev" branch on -rcu that will make this function appear
> > as shown below.
> >
> > So what would you use that pattern for?
> >
> > One possibility is a per-CPU statistical counter in userspace on a
> > fastpath, in cases where losing the occasional count is OK. Then learning
> > your CPU (and possibly being immediately migrated to some other CPU),
> > READ_ONCE() of the count, increment, and WRITE_ONCE() might (or might not)
> > make sense.
> >
> > I suppose the same in the kernel if there was a fastpath so extreme you
> > could not afford to disable preemption.
> >
> > At best, very niche.
> >
> > Or am I suffering a failure of imagination yet again? ;-)
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > static bool
> > rcu_torture_pipe_update_one(struct rcu_torture *rp)
> > {
> > int i;
> > struct rcu_torture_reader_check *rtrcp = READ_ONCE(rp->rtort_chkp);
> >
> > if (rtrcp) {
> > WRITE_ONCE(rp->rtort_chkp, NULL);
> > smp_store_release(&rtrcp->rtc_ready, 1); // Pair with smp_load_acquire().
> > }
> > i = rp->rtort_pipe_count;
> > if (i > RCU_TORTURE_PIPE_LEN)
> > i = RCU_TORTURE_PIPE_LEN;
> > atomic_inc(&rcu_torture_wcount[i]);
> > WRITE_ONCE(rp->rtort_pipe_count, i + 1);
> > ASSERT_EXCLUSIVE_WRITER(rp->rtort_pipe_count);
>
> I was going to say to add a comment here for the future reader, that update-side
> ->rtort_pipe_count READ/WRITE are already mutually exclusive, but this ASSERT
> already documents it ;-)
Plus KCSAN is way better at repeatedly inspecting code for this sort of
issue than I am. ;-)
> Also FWIW I confirmed after starting at code that indeed only one update-side
> access is possible at a time! Thanks,
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Thank you very much! I will apply your Reviewed-by to these commits
on my next rebase:
28455c73b620 ("rcutorture: ASSERT_EXCLUSIVE_WRITER() for ->rtort_pipe_count updates")
b0b99e7db12e ("rcutorture: Remove extraneous rcu_torture_pipe_update_one() READ_ONCE()")
Thanx, Paul
