[dpdk-dev] [RFC] eal: simplify the implementation of rte_ctrl_thread_create

[Honnappa Nagarahalli]

<snip>

> 
> Hi Honnappa,
> 
> (Sorry if you get this message twice, I forgot to reply all the first time)
> 
> Sorry for the late reply. I was also away.
No problem, thanks for your comments.

> 
> I have only made one small contribution to DPDK so I'll defer to others to
> decide whether this patch should be accepted. When I submitted my patch, I
> got the feeling that busy loops were somewhat frowned upon but it might be
> a good solution here.
IMO, it should be ok here since the busy loop is for a small period. Adding sched_yield should help reduce the actual spinning.

> 
> Le ven. 30 juil. 2021 à 17:37, Honnappa Nagarahalli
> <honnappa.nagarahalli at arm.com> a écrit :
> <snip>
> > +       /* Synchronization variable between the control thread
> > +        * and the thread calling rte_ctrl_thread_create function.
> > +        * 0 - Initialized
> > +        * 1 - Control thread is running successfully
> > +        * 2 - Control thread encountered an error. 'ret' has the
> > +        *     error code.
> > +        */
> > +       unsigned int sync;
> 
> This is highly subjective but the name of this variable doesn't clearly indicate
> that once it is set, the control thread is effectively done with the
> rte_thread_ctrl_params. The downside that I can see (as opposed to the
> previous reference counting approach) is that anyone making further changes
> would have to be cognizant of that fact. Otherwise, any new code that
> accesses "sync" in the control thread, after "sync" has been set, would create
> a race.
May be "ctrl_thread_status" would be better.

> 
> <snip>
> >         ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
> >         if (ret != 0)
> > -               goto fail_with_barrier;
> > +               goto thread_create_failed;
> 
> This is also highly subjective but since the goto label is only used here, you
> could completely get rid of the goto and simply free and return here.
Thanks, agree.

> 
> <snip>
> > +       /* Wait for the control thread to initialize successfully */
> > +       while (!params->sync)
> > +               rte_pause();
> > +       ret = params->ret;
> >
> > -       pthread_barrier_wait(&params->configured);
> > -       ctrl_params_free(params);
> > +       free(params);
> 
> Maybe I'm missing something but it seems like you're accessing
> params->sync after this line (use after free). I assume you meant to
> add this line after the subsequent if block?
You are correct. I realized this yesterday while incorporating review comments. That made me think, we need a test case. I am adding a test case as well.

> 
> Le ven. 30 juil. 2021 à 17:37, Honnappa Nagarahalli
> <honnappa.nagarahalli at arm.com> a écrit :
> >
> > The current described behaviour of rte_ctrl_thread_create is rigid
> > which makes the implementation of the function complex.
> > The behavior is abstracted to allow for simplified implementation.
> >
> > Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> > ---
> >  lib/eal/common/eal_common_thread.c | 65 +++++++++++++-----------------
> >  lib/eal/include/rte_lcore.h        |  8 ++--
> >  2 files changed, 32 insertions(+), 41 deletions(-)
> >
> > diff --git a/lib/eal/common/eal_common_thread.c
> > b/lib/eal/common/eal_common_thread.c
> > index 1a52f42a2b..86cacd840c 100644
> > --- a/lib/eal/common/eal_common_thread.c
> > +++ b/lib/eal/common/eal_common_thread.c
> > @@ -169,35 +169,35 @@ __rte_thread_uninit(void)  struct
> > rte_thread_ctrl_params {
> >         void *(*start_routine)(void *);
> >         void *arg;
> > -       pthread_barrier_t configured;
> > -       unsigned int refcnt;
> > +       int ret;
> > +       /* Synchronization variable between the control thread
> > +        * and the thread calling rte_ctrl_thread_create function.
> > +        * 0 - Initialized
> > +        * 1 - Control thread is running successfully
> > +        * 2 - Control thread encountered an error. 'ret' has the
> > +        *     error code.
> > +        */
> > +       unsigned int sync;
> >  };
> >
> > -static void ctrl_params_free(struct rte_thread_ctrl_params *params)
> > -{
> > -       if (__atomic_sub_fetch(&params->refcnt, 1, __ATOMIC_ACQ_REL) ==
> 0) {
> > -               (void)pthread_barrier_destroy(&params->configured);
> > -               free(params);
> > -       }
> > -}
> > -
> >  static void *ctrl_thread_init(void *arg)  {
> >         struct internal_config *internal_conf =
> >                 eal_get_internal_configuration();
> >         rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset;
> >         struct rte_thread_ctrl_params *params = arg;
> > -       void *(*start_routine)(void *);
> > +       void *(*start_routine)(void *) = params->start_routine;
> >         void *routine_arg = params->arg;
> >
> >         __rte_thread_init(rte_lcore_id(), cpuset);
> > -
> > -       pthread_barrier_wait(&params->configured);
> > -       start_routine = params->start_routine;
> > -       ctrl_params_free(params);
> > -
> > -       if (start_routine == NULL)
> > +       params->ret = pthread_setaffinity_np(pthread_self(),
> > +                               sizeof(*cpuset), cpuset);
> > +       if (params->ret != 0) {
> > +               params->sync = 2;
> >                 return NULL;
> > +       }
> > +
> > +       params->sync = 1;
> >
> >         return start_routine(routine_arg);  } @@ -207,9 +207,6 @@
> > rte_ctrl_thread_create(pthread_t *thread, const char *name,
> >                 const pthread_attr_t *attr,
> >                 void *(*start_routine)(void *), void *arg)  {
> > -       struct internal_config *internal_conf =
> > -               eal_get_internal_configuration();
> > -       rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset;
> >         struct rte_thread_ctrl_params *params;
> >         int ret;
> >
> > @@ -219,15 +216,12 @@ rte_ctrl_thread_create(pthread_t *thread, const
> > char *name,
> >
> >         params->start_routine = start_routine;
> >         params->arg = arg;
> > -       params->refcnt = 2;
> > -
> > -       ret = pthread_barrier_init(&params->configured, NULL, 2);
> > -       if (ret != 0)
> > -               goto fail_no_barrier;
> > +       params->ret = 0;
> > +       params->sync = 0;
> >
> >         ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
> >         if (ret != 0)
> > -               goto fail_with_barrier;
> > +               goto thread_create_failed;
> >
> >         if (name != NULL) {
> >                 ret = rte_thread_setname(*thread, name); @@ -236,24
> > +230,21 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
> >                                 "Cannot set name for ctrl thread\n");
> >         }
> >
> > -       ret = pthread_setaffinity_np(*thread, sizeof(*cpuset), cpuset);
> > -       if (ret != 0)
> > -               params->start_routine = NULL;
> > +       /* Wait for the control thread to initialize successfully */
> > +       while (!params->sync)
> > +               rte_pause();
> > +       ret = params->ret;
> >
> > -       pthread_barrier_wait(&params->configured);
> > -       ctrl_params_free(params);
> > +       free(params);
> >
> > -       if (ret != 0)
> > -               /* start_routine has been set to NULL above; */
> > -               /* ctrl thread will exit immediately */
> > +       if (params->sync != 1)
> > +               /* ctrl thread is exiting */
> >                 pthread_join(*thread, NULL);
> >
> >         return -ret;
> >
> > -fail_with_barrier:
> > -       (void)pthread_barrier_destroy(&params->configured);
> > +thread_create_failed:
> >
> > -fail_no_barrier:
> >         free(params);
> >
> >         return -ret;
> > diff --git a/lib/eal/include/rte_lcore.h b/lib/eal/include/rte_lcore.h
> > index 1550b75da0..f1cc5e38dc 100644
> > --- a/lib/eal/include/rte_lcore.h
> > +++ b/lib/eal/include/rte_lcore.h
> > @@ -420,10 +420,10 @@ rte_thread_unregister(void);
> >  /**
> >   * Create a control thread.
> >   *
> > - * Wrapper to pthread_create(), pthread_setname_np() and
> > - * pthread_setaffinity_np(). The affinity of the new thread is based
> > - * on the CPU affinity retrieved at the time rte_eal_init() was
> > called,
> > - * the dataplane and service lcores are then excluded.
> > + * Creates a control thread with the given name and attributes. The
> > + * affinity of the new thread is based on the CPU affinity retrieved
> > + * at the time rte_eal_init() was called, the dataplane and service
> > + * lcores are then excluded.
> >   *
> >   * @param thread
> >   *   Filled with the thread id of the new created thread.
> > --
> > 2.17.1
> >