On 14.05.2019 10:38, David Marchand wrote: > Hello Ilya, > On Mon, May 6, 2019 at 5:37 PM Ilya Maximets <i.maxim...@samsung.com > <mailto:i.maxim...@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.march...@redhat.com > <mailto:david.march...@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 Best regards, Ilya Maximets. _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev
