Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

[Joel Fernandes]

On Thu, Dec 22, 2022 at 2:45 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Dec 22, 2022 at 01:56:17PM -0500, Joel Fernandes wrote:
> >
> >
> > > On Dec 22, 2022, at 1:53 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Dec 22, 2022 at 01:19:06PM -0500, Joel Fernandes wrote:
> > >>
> > >>
> > >>>> On Dec 22, 2022, at 11:43 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >>>
> > >>> On Thu, Dec 22, 2022 at 01:40:10PM +0100, Frederic Weisbecker wrote:
> > >>>>> On Wed, Dec 21, 2022 at 12:11:42PM -0500, Mathieu Desnoyers wrote:
> > >>>>> On 2022-12-21 06:59, Frederic Weisbecker wrote:
> > >>>>>>> On Tue, Dec 20, 2022 at 10:34:19PM -0500, Mathieu Desnoyers wrote:
> > >>>>> [...]
> > >>>>>>>
> > >>>>>>> The memory ordering constraint I am concerned about here is:
> > >>>>>>>
> > >>>>>>> * [...] In addition,
> > >>>>>>> * each CPU having an SRCU read-side critical section that extends beyond
> > >>>>>>> * the return from synchronize_srcu() is guaranteed to have executed a
> > >>>>>>> * full memory barrier after the beginning of synchronize_srcu() and before
> > >>>>>>> * the beginning of that SRCU read-side critical section. [...]
> > >>>>>>>
> > >>>>>>> So if we have a SRCU read-side critical section that begins after the beginning
> > >>>>>>> of synchronize_srcu, but before its first memory barrier, it would miss the
> > >>>>>>> guarantee that the full memory barrier is issued before the beginning of that
> > >>>>>>> SRCU read-side critical section. IOW, that memory barrier needs to be at the
> > >>>>>>> very beginning of the grace period.
> > >>>>>>
> > >>>>>> I'm confused, what's wrong with this ?
> > >>>>>>
> > >>>>>> UPDATER                  READER
> > >>>>>> -------                  ------
> > >>>>>> STORE X = 1              STORE srcu_read_lock++
> > >>>>>> // rcu_seq_snap()        smp_mb()
> > >>>>>> smp_mb()                 READ X
> > >>>>>> // scans
> > >>>>>> READ srcu_read_lock
> > >>>>>
> > >>>>> What you refer to here is only memory ordering of the store to X and load
> > >>>>> from X wrt loading/increment of srcu_read_lock, which is internal to the
> > >>>>> srcu implementation. If we really want to model the provided high-level
> > >>>>> memory ordering guarantees, we should consider a scenario where SRCU is used
> > >>>>> for its memory ordering properties to synchronize other variables.
> > >>>>>
> > >>>>> I'm concerned about the following Dekker scenario, where synchronize_srcu()
> > >>>>> and srcu_read_lock/unlock would be used instead of memory barriers:
> > >>>>>
> > >>>>> Initial state: X = 0, Y = 0
> > >>>>>
> > >>>>> Thread A                   Thread B
> > >>>>> ---------------------------------------------
> > >>>>> STORE X = 1                STORE Y = 1
> > >>>>> synchronize_srcu()
> > >>>>>                          srcu_read_lock()
> > >>>>>                          r1 = LOAD X
> > >>>>>                          srcu_read_unlock()
> > >>>>> r0 = LOAD Y
> > >>>>>
> > >>>>> BUG_ON(!r0 && !r1)
> > >>>>>
> > >>>>> So in the synchronize_srcu implementation, there appears to be two
> > >>>>> major scenarios: either srcu_gp_start_if_needed starts a gp or expedited gp,
> > >>>>> or it uses an already started gp/expedited gp. When snapshotting with
> > >>>>> rcu_seq_snap, the fact that the memory barrier is after the ssp->srcu_gp_seq
> > >>>>> load means that it does not order prior memory accesses before that load.
> > >>>>> This sequence value is then used to identify which gp_seq to wait for when
> > >>>>> piggy-backing on another already-started gp. I worry about reordering
> > >>>>> between STORE X = 1 and load of ssp->srcu_gp_seq, which is then used to
> > >>>>> piggy-back on an already-started gp.
> > >>>>>
> > >>>>> I suspect that the implicit barrier in srcu_read_lock() invoked at the
> > >>>>> beginning of srcu_gp_start_if_needed() is really the barrier that makes
> > >>>>> all this behave as expected. But without documentation it's rather hard to
> > >>>>> follow.
> > >>>>
> > >>>> Oh ok I see now. It might be working that way by accident or on forgotten
> > >>>> purpose. In any case, we really want to add a comment above that
> > >>>> __srcu_read_lock_nmisafe() call.
> > >>>
> > >>> Another test for the safety (or not) of removing either D or E is
> > >>> to move that WRITE_ONCE() to follow (or, respectively, precede) the
> > >>> adjacent scans.
> > >>
> > >> Good idea, though I believe the MBs that the above talk about are not the flip ones. They are the ones in synchronize_srcu() beginning and end, that order with respect to grace period start and end.
> > >>
> > >> So that (flipping MBs) is unrelated, or did I miss something?
> > >
> > > The thought is to manually similate in the source code the maximum
> > > memory-reference reordering that a maximally hostile compiler and CPU
> > > would be permitted to carry out.  So yes, given that there are other
> > > memory barriers before and after, these other memory barriers limit how
> > > far the flip may be moved in the source code.
> > >
> > > Here I am talking about the memory barriers associated with the flip,
> > > but the same trick can of course be applied to other memory barriers.
> > > In general, remove a given memory barrier and (in the source code)
> > > maximally rearrange the memory references that were previously ordered
> > > by the memory barrier in question.
> > >
> > > Again, the presence of other memory barriers will limit the permitted
> > > maximal source-code rearrangement.
> >
> >
> > Makes sense if the memory barrier is explicit. In this case, the memory barriers are implicit apparently, with a srcu_read_lock() in the beginning of synchronize_rcu() having the implicit / indirect memory barrier. So I am not sure if that can be implemented without breaking SRCU readers.
>
> First, are we talking about the same barrier?  I am talking about E.

No, in the last part you replied to above, Mathieu and Frederic were
talking about the need for memory barriers in synchronize_srcu(). That
has nothing to do with E:
<quote>
 I suspect that the implicit barrier in srcu_read_lock() invoked at the
 beginning of srcu_gp_start_if_needed() is really the barrier that makes
 all this behave as expected.
</quote>

We need to order code prior to synchronize_srcu() wrt the start of the
grace period, so that readers that started after the grace period
started see those side-effects as they may not be waited on (they are
too late).

> Alternatively, remove E and hammer it on a weakly ordered system.

But E is useless, see below patch link with litmus test [1]. I don't
see what hammering it on a weakly ordered system buys if it does not
even execute for the cases when it is supposed to help because of
causality with the control dependency of the prior scan :-/

Thanks,

  -Joel
[1] https://lore.kernel.org/rcu/19A03A16-9F38-481F-824D-4C883AB8B105@xxxxxxxxxxxxxxxxx/T/#m60e37d1277a8dd626bb95f92083e4e602c429704