Re: [PATCH v2] ring: enforce reading the tails before ring operations

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> Hi Gavin,
> 
> > > >>> In weak memory models, like arm64, reading the {prod,cons}.tail
> > > >>> may get reordered after reading or writing the ring slots, which
> > > >>> corrupts the ring and stale data is observed.
> > > >>>
> > > >>> This issue was reported by NXP on 8-A72 DPAA2 board. The problem
> > > >>> is
> > > >> most
> > > >>> likely caused by missing the acquire semantics when reading
> > > >>> cons.tail (in SP enqueue) or prod.tail (in SC dequeue) which
> > > >>> makes it possible to
> > > read
> > > >> a
> > > >>> stale value from the ring slots.
> > > >>>
> > > >>> For MP (and MC) case, rte_atomic32_cmpset() already provides the
> > > >> required
> > > >>> ordering. This patch is to prevent reading and writing the ring
> > > >>> slots get reordered before reading {prod,cons}.tail for SP (and SC)
> case.
> > > >>
> > > >> Read barrier rte_smp_rmb() is OK to prevent reading the ring get
> > > >> reordered before reading the tail. However, to prevent *writing*
> > > >> the ring get reordered *before reading* the tail you need a full
> > > >> memory barrier, i.e.
> > > >> rte_smp_mb().
> > > >
> > > > ISHLD(rte_smp_rmb is DMB(ishld) orders LD/LD and LD/ST, while
> > > > WMB(ST
> > > Option) orders ST/ST.
> > > > For more details, please refer to: Table B2-1 Encoding of the DMB
> > > > and DSB
> > > <option> parameter  in
> > > > https://developer.arm.com/docs/ddi0487/latest/arm-architecture-
> > > reference-manual-armv8-for-armv8-a-architecture-profile
> > >
> > > I see. But you have to change the rte_smp_rmb() function definition
> > > in lib/librte_eal/common/include/generic/rte_atomic.h and assure
> > > that all other architectures follows same rules.
> > > Otherwise, this change is logically wrong, because read barrier in
> > > current definition could not be used to order Load with Store.
> > >
> >
> > Good points, let me re-think how to handle for other architectures.
> > Full MB is required for other architectures(x86? Ppc?), but for arm, read
> barrier(load/store and load/load) is enough.
> 
> For x86, I don't think you need any barrier here, as with IA memory mode:
> -  Reads are not reordered with other reads.
> - Writes are not reordered with older reads.
Agree

> 
> BTW, could you explain a bit more why barrier is necessary even on arm here?
> As I can see, there is a data dependency between the tail value and
> subsequent address calculations for ring writes/reads.
> Isn't that sufficient to prevent re-ordering even for weak memory model?
The tail value affects 'n'. But, the value of 'n' can be speculated because of the following 'if' statement:

if (unlikely(n > *free_entries))
                        n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 : *free_entries;

The address calculations for actual ring writes/reads do not depend on the tail value. Since 'n' can be speculated, the writes/reads can be moved up before the load of the tail value.

> Konstantin
> 
> 
<snip>