On Tue, May 14, 2019 at 11:53 AM Ilya Maximets <[email protected]> wrote:
> > On 14.05.2019 10:38, David Marchand wrote: > > Hello Ilya, > > On Mon, May 6, 2019 at 5:37 PM Ilya Maximets <[email protected] > <mailto:[email protected]>> 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 <[email protected] <mailto: > [email protected]>> > > > --- > > > 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 _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
