Re: [dpdk-dev] [PATCH 1/3] eventdev: fix race condition on timer list counter

[Phil Yang <Phil.Yang@arm.com>]

> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Sent: Friday, June 19, 2020 2:26 AM
> To: Carrillo, Erik G <erik.g.carrillo@intel.com>; Phil Yang
> <Phil.Yang@arm.com>; dev@dpdk.org
> Cc: drc@linux.vnet.ibm.com; Ruifeng Wang <Ruifeng.Wang@arm.com>;
> Dharmik Thakkar <Dharmik.Thakkar@arm.com>; nd <nd@arm.com>;
> stable@dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> nd <nd@arm.com>
> Subject: RE: [PATCH 1/3] eventdev: fix race condition on timer list counter
> 
> <snip>
> 
> >
> > Hi Phil,
> >
> > Good catch - thanks for the fix.   I've commented in-line:
> >
> > > -----Original Message-----
> > > From: Phil Yang <phil.yang@arm.com>
> > > Sent: Friday, June 12, 2020 6:20 AM
> > > To: dev@dpdk.org; Carrillo, Erik G <erik.g.carrillo@intel.com>
> > > Cc: drc@linux.vnet.ibm.com; honnappa.nagarahalli@arm.com;
> > > ruifeng.wang@arm.com; dharmik.thakkar@arm.com; nd@arm.com;
> > > stable@dpdk.org
> > > Subject: [PATCH 1/3] eventdev: fix race condition on timer list
> > > counter
> > >
> > > The n_poll_lcores counter and poll_lcore array are shared between
> > > lcores and the update of these variables are out of the protection of
> > > spinlock on each lcore timer list. The read-modify-write operations of
> > > the counter are not atomic, so it has the potential of race condition
> > between lcores.
> > >
> > > Use c11 atomics with RELAXED ordering to prevent confliction.
> > >
> > > Fixes: cc7b73ea9e3b ("eventdev: add new software timer adapter")
> > > Cc: erik.g.carrillo@intel.com
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > ---
> > >  lib/librte_eventdev/rte_event_timer_adapter.c | 16 ++++++++++++----
> > >  1 file changed, 12 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c
> > > b/lib/librte_eventdev/rte_event_timer_adapter.c
> > > index 005459f..6a0e283 100644
> > > --- a/lib/librte_eventdev/rte_event_timer_adapter.c
> > > +++ b/lib/librte_eventdev/rte_event_timer_adapter.c
> > > @@ -583,6 +583,7 @@ swtim_callback(struct rte_timer *tim)
> > >  uint16_t nb_evs_invalid = 0;
> > >  uint64_t opaque;
> > >  int ret;
> > > +int n_lcores;
> > >
> > >  opaque = evtim->impl_opaque[1];
> > >  adapter = (struct rte_event_timer_adapter *)(uintptr_t)opaque; @@
> > > -605,8 +606,12 @@ swtim_callback(struct rte_timer *tim)
> > >        "with immediate expiry value");
> > >  }
> > >
> > > -if (unlikely(rte_atomic16_test_and_set(&sw-
> > > >in_use[lcore].v)))
> > > -sw->poll_lcores[sw->n_poll_lcores++] = lcore;
> > > +if (unlikely(rte_atomic16_test_and_set(&sw-
> > > >in_use[lcore].v))) {
> > > +n_lcores = __atomic_fetch_add(&sw->n_poll_lcores,
> > > 1,
> > > +__ATOMIC_RELAXED);
> Since this commit will be back ported, we should prefer to use rte_atomic
> APIs for this commit. Otherwise, we will have a mix of rte_atomic and C11
> APIs.
> My suggestion is to fix this bug using rte_atomic so that backported code will
> have only rte_atomic APIs. Add another commit (if required) in this series to
> make the bug fix use C11 APIs (this commit will not be backported).

Agree. 
I will change this patch to the rte_atomic version in the next version.

Thanks,
Phil

> 
> >
> > Just a nit, but let's align the continued line with the opening parentheses in
> > this location and below.  With these changes:
> >
> > Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
> >
> > > +__atomic_store_n(&sw->poll_lcores[n_lcores],
> > > lcore,
> > > +__ATOMIC_RELAXED);
> > > +}
> > >  } else {
> > >  EVTIM_BUF_LOG_DBG("buffered an event timer expiry event");
> > >
> > > @@ -1011,6 +1016,7 @@ __swtim_arm_burst(const struct
> > > rte_event_timer_adapter *adapter,
> > >  uint32_t lcore_id = rte_lcore_id();
> > >  struct rte_timer *tim, *tims[nb_evtims];
> > >  uint64_t cycles;
> > > +int n_lcores;
> > >
> > >  #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
> > >  /* Check that the service is running. */ @@ -1033,8 +1039,10 @@
> > > __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
> > >  if (unlikely(rte_atomic16_test_and_set(&sw->in_use[lcore_id].v))) {
> > >  EVTIM_LOG_DBG("Adding lcore id = %u to list of lcores to
> > poll",
> > >        lcore_id);
> > > -sw->poll_lcores[sw->n_poll_lcores] = lcore_id;
> > > -++sw->n_poll_lcores;
> > > +n_lcores = __atomic_fetch_add(&sw->n_poll_lcores, 1,
> > > +__ATOMIC_RELAXED);
> > > +__atomic_store_n(&sw->poll_lcores[n_lcores], lcore_id,
> > > +__ATOMIC_RELAXED);
> > >  }
> > >
> > >  ret = rte_mempool_get_bulk(sw->tim_pool, (void **)tims,
> > > --
> > > 2.7.4
>