Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer() data race and concurrency bug

[Mathieu Desnoyers]

On 2024-03-07 14:47, Paul E. McKenney wrote:
> On Thu, Mar 07, 2024 at 08:53:05AM -0500, Mathieu Desnoyers wrote:
> > On 2024-03-06 22:37, Paul E. McKenney wrote:
> > > On Wed, Mar 06, 2024 at 10:06:21PM -0500, Mathieu Desnoyers wrote:
> > [...]
> > > > 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().
> > [...]
> > > 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?  ;-)
> >
> > The (niche) use-cases I have in mind are split-counters and RCU
> > grace period tracking, where precise counters totals are needed
> > (no lost count).
> >
> > In the kernel, this could be:
>
> Thank you for looking into this!
>
> > - A per-cpu counter, each counter incremented from thread context with
> >    preemption disabled (single updater per counter), read concurrently by
> >    other threads. WRITE_ONCE/READ_ONCE is useful to make sure there
> >    is no store/load tearing there. Atomics on the update would be stronger
> >    than necessary on the increment fast-path.
>
> But if preemption is disabled, the updater can read the value without
> READ_ONCE() without risk of concurrent update.  Or are you concerned about
> interrupt handlers?  This would have to be a read from the interrupt
> handler, given that an updated from the interrupt handler could result
> in a lost count.

You are correct that the updater don't need READ_ONCE there. It would
however require a WRITE_ONCE() to match READ_ONCE() from concurrent
reader threads.

> > - A per-thread counter (e.g. within task_struct), only incremented by the
> >    single thread, read by various threads concurrently.
>
> Ditto.

Right, only WRITE_ONCE() on the single updater, READ_ONCE() on readers.

> > - A counter which increment happens to be already protected by a lock, read
> >    by various threads without taking the lock. (technically doable, but
> >    I'm not sure I see a relevant use-case for it)
>
> In that case, the lock would exclude concurrent updates, so the lock
> holder would not need READ_ONCE(), correct?

Correct.

> > In user-space:
> >
> > - The "per-cpu" counter would have to use rseq for increments to prevent
> >    inopportune migrations, which needs to be implemented in assembler anyway.
> >    The counter reads would have to use READ_ONCE().
>
> Fair enough!
>
> > - The per-thread counter (Thread-Local Storage) incremented by a single
> >    thread, read by various threads concurrently, is a good target
> >    for WRITE_ONCE()/READ_ONCE() pairing. This is actually what we do in
> >    various liburcu implementations which track read-side critical sections
> >    per-thread.
>
> Agreed, but do any of these use WRITE_ONCE(x, READ_ONCE(x) + 1) or
> similar?

Not quite, I recall it's more like WRITE_ONCE(x, READ_ONCE(y)) or such,
so we can grab the value of the current gp counter and store it into a
TLS variable.


> > - Same as for the kernel, a counter increment protected by a lock which
> >    needs to be read from various threads concurrently without taking
> >    the lock could be a valid use-case, though I fail to see how it is
> >    useful due to lack of imagination on my part. ;-)
>
> In RCU, we have "WRITE_ONCE(*sp, *sp + 1)" for this use case, though
> here we have the WRITE_ONCE() but not the READ_ONCE() because we hold
> the lock excluding any other updates.

You are right, the READ_ONCE() is not needed in this case for the
updater, only for the concurrent readers.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com