[ovs-dev] [RFC 1/5] dpif-netdev: Convert exit latch to flag.

David Marchand david.marchand at redhat.com
Tue May 14 10:04:16 UTC 2019


On Tue, May 14, 2019 at 11:53 AM Ilya Maximets <i.maximets at samsung.com>
wrote:

>
> On 14.05.2019 10:38, David Marchand wrote:
> > Hello Ilya,
> > On Mon, May 6, 2019 at 5:37 PM Ilya Maximets <i.maximets at samsung.com
> <mailto:i.maximets at samsung.com>> wrote:
> >
> >     On 30.04.2019 15:17, David Marchand wrote:
> >     > No need for a latch here since we don't have to wait.
> >     > A simple boolean flag is enough.
> >     >
> >     > Fixes: e4cfed38b159 ("dpif-netdev: Add poll-mode-device thread.")
> >     > Signed-off-by: David Marchand <david.marchand at redhat.com <mailto:
> david.marchand at redhat.com>>
> >     > ---
> >     >  lib/dpif-netdev.c | 9 ++++-----
> >     >  1 file changed, 4 insertions(+), 5 deletions(-)
> >     >
> >     > diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> >     > index f1422b2..30774ed 100644
> >     > --- a/lib/dpif-netdev.c
> >     > +++ b/lib/dpif-netdev.c
> >     > @@ -681,10 +681,10 @@ struct dp_netdev_pmd_thread {
> >     >      /* Current context of the PMD thread. */
> >     >      struct dp_netdev_pmd_thread_ctx ctx;
> >     >
> >     > -    struct latch exit_latch;        /* For terminating the pmd
> thread. */
> >     >      struct seq *reload_seq;
> >     >      uint64_t last_reload_seq;
> >     >      atomic_bool reload;             /* Do we need to reload
> ports? */
> >     > +    atomic_bool exit;               /* For terminating the pmd
> thread. */
> >     >      pthread_t thread;
> >     >      unsigned core_id;               /* CPU core id of this pmd
> thread. */
> >     >      int numa_id;                    /* numa node id of this pmd
> thread. */
> >     > @@ -5479,7 +5479,7 @@ reload:
> >     >      ovs_mutex_unlock(&pmd->perf_stats.stats_mutex);
> >     >
> >     >      poll_cnt = pmd_load_queues_and_ports(pmd, &poll_list);
> >     > -    exiting = latch_is_set(&pmd->exit_latch);
> >     > +    atomic_read_relaxed(&pmd->exit, &exiting);
> >
> >     I'm afraid that relaxed memory model is not suitable here.
> >     You need to change memory models for both 'reload' and 'exit'
> >     or put ack_rel thread fence between them. Otherwise reads/writes
> >     could be reordered resulting in missed exit and main thread
> >     hang on join.
> >
> >
> > Indeed, I can not use relaxed memory model on both atomics.
> >
> > I have been reading some articles and I am a bit puzzled :-).
> > I don't understand why I would need to update both atomics memory model.
> >
> > On the pmd thread side:
> > atomic_read_explicit(&pmd->reload, &reload, memory_model_acquire);
> > atomic_read_relaxed(&pmd->exit, &exiting);
> >
> > On the control side:
> > atomic_store_relaxed(&pmd->exit, true);
> > atomic_store_explicit(&pmd->reload, true, memory_order_release);
> >
> > Would not it be enough to have those threads share the same view by
> synchronising on reload ?
>
> Yes. You're right. Above example is valid.
> I re-checked the spec for release-acquire ordering and it seems that
> we could use 'reload' with rel-acq ordering as a synchronization point
> because it will force all memory writes (non-atomic and relaxed atomic)
> that happened-before the rel atomic store in main thread become visible
> side-effects in PMD thread after the acq atomic read.
>
> Probably, I was confused because of thinking that we need a backward
> synchronization (PMD --> main). But we don't.
>
>
> https://en.cppreference.com/w/c/atomic/memory_order#Release-Acquire_ordering



Thanks a lot.
Then I will revisit my series with that in mind.


-- 
David Marchand


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