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

Paul E. McKenney
2022-12-22T16:43:11Z

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.

Thanx, Paul