netdev - Re: [PATCH net-next] dql: annotate data-races around dql->last_obj

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Message-ID: <CANn89iKFbtMM2XvXdFDi1hrCn8UdRdPsuhZ0Js7Vo61e__ULpw@mail.gmail.com>
Date: Tue, 29 Oct 2024 20:39:31 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Joe Damato <jdamato@...tly.com>, Eric Dumazet <edumazet@...gle.com>, 
	"David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] dql: annotate data-races around dql->last_obj_cnt

On Tue, Oct 29, 2024 at 8:34 PM Joe Damato <jdamato@...tly.com> wrote:
>
> On Tue, Oct 29, 2024 at 07:14:25PM +0000, Eric Dumazet wrote:
> > dql->last_obj_cnt is read/written from different contexts,
> > without any lock synchronization.
> >
> > Use READ_ONCE()/WRITE_ONCE() to avoid load/store tearing.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > ---
> >  include/linux/dynamic_queue_limits.h | 2 +-
> >  lib/dynamic_queue_limits.c           | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/dynamic_queue_limits.h b/include/linux/dynamic_queue_limits.h
> > index 281298e77a1579cba1f92a3b3f03b8be089fd38f..808b1a5102e7c0bbbcd9676b0dacadad2f0ee49a 100644
> > --- a/include/linux/dynamic_queue_limits.h
> > +++ b/include/linux/dynamic_queue_limits.h
> > @@ -127,7 +127,7 @@ static inline void dql_queued(struct dql *dql, unsigned int count)
> >       if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
> >               return;
> >
> > -     dql->last_obj_cnt = count;
> > +     WRITE_ONCE(dql->last_obj_cnt, count);
> >
> >       /* We want to force a write first, so that cpu do not attempt
> >        * to get cache line containing last_obj_cnt, num_queued, adj_limit
> > diff --git a/lib/dynamic_queue_limits.c b/lib/dynamic_queue_limits.c
> > index e49deddd3de9fe9e98d6712559cf48d12a0a2537..c1b7638a594ac43f947e00decabbd3468dcb53de 100644
> > --- a/lib/dynamic_queue_limits.c
> > +++ b/lib/dynamic_queue_limits.c
> > @@ -179,7 +179,7 @@ void dql_completed(struct dql *dql, unsigned int count)
> >
> >       dql->adj_limit = limit + completed;
> >       dql->prev_ovlimit = ovlimit;
> > -     dql->prev_last_obj_cnt = dql->last_obj_cnt;
> > +     dql->prev_last_obj_cnt = READ_ONCE(dql->last_obj_cnt);
> >       dql->num_completed = completed;
> >       dql->prev_num_queued = num_queued;
> >
>
> This looks fine to me. I noted that dql_reset writes last_obj_cnt,
> but AFAIU that write is not a problem (from the 1 driver I looked
> at).

Yeah, I think that dql_reset() should not exist in the first place.

When all skbs are properly tx completed, BQL state should be known and clean.