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

Joel Fernandes
2022-12-23T16:12:27Z

On Thu, Dec 22, 2022 at 11:43 PM Joel Fernandes <joel@joelfernandes.org> wrote:
[...]
> > > >>>> On Dec 22, 2022, at 11:43 AM, Paul E. McKenney <paulmck@kernel.org> 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).

My thought is this is achieved by the srcu_read_lock() before the
grace period is started, and the start of the grace period (which is
basically the smp_mb() in the first scan).

So from memory ordering PoV, if synchronize_rcu() spans the GP, and
readers don't span the GP, that means the reader does not span the
synchronize_rcu() which is the GP guarantee.

But I could be missing something. I will dig more on my side. Thanks.