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

Frederic Weisbecker
2022-12-22T12:40:21Z

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.