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

Paul E. McKenney
2022-12-22T19:45:22Z

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@kernel.org> 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@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.

Yes, this would require a bit of restructuring. The overall
approach would be something like this, in SRCU_STATE_SCAN1:

1. Scan the unlocks.

2. smp_mb(); /* A */

3. Flip the index.

4. Scan the locks.

5. If unlocks == locks, advance the state to SRCU_STATE_SCAN2.

6. Otherwise, execute the current SRCU_STATE_SCAN1 code.

Give or take the usual devils in the details.

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

Thanx, Paul