linux-kernel - Re: [PATCH net] net: fix premature exit from NAPI state polling in napi

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Message-ID: <20211110174453.3cb33cde@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 10 Nov 2021 17:44:53 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Michal Swiatkowski <michal.swiatkowski@...el.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>,
        Björn Töpel <bjorn@...nel.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: fix premature exit from NAPI state polling in
 napi_disable()

On Wed, 10 Nov 2021 17:44:07 -0800 Jakub Kicinski wrote:
> On Wed, 10 Nov 2021 20:43:37 +0100 Alexander Lobakin wrote:
> > > What about replacing the error prone do {...} while (cmpxchg(..)) by
> > > something less confusing ?
> > > 
> > > This way, no need for a label.
> > > 
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 5e37d6809317fb3c54686188a908bfcb0bfccdab..9327141892cdaaf0282e082e0c6746abae0f12a7
> > > 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -6264,7 +6264,7 @@ void napi_disable(struct napi_struct *n)
> > >         might_sleep();
> > >         set_bit(NAPI_STATE_DISABLE, &n->state);
> > > 
> > > -       do {
> > > +       for (;;) {
> > >                 val = READ_ONCE(n->state);
> > >                 if (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) {
> > >                         usleep_range(20, 200);
> > > @@ -6273,7 +6273,9 @@ void napi_disable(struct napi_struct *n)
> > > 
> > >                 new = val | NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC;
> > >                 new &= ~(NAPIF_STATE_THREADED | NAPIF_STATE_PREFER_BUSY_POLL);
> > > -       } while (cmpxchg(&n->state, val, new) != val);
> > > +               if (cmpxchg(&n->state, val, new) == val)
> > > +                       break;
> > > +       }
> > > 
> > >         hrtimer_cancel(&n->timer);    
> > 
> > LFTM, I'l queue v2 in a moment with you in Suggested-by.  
> 
> Ouch.
> 
> Feel free to put
> 
> Acked-by: Jakub Kicinski <kuba@...nel.org>

Or I'll do it myself since it's already out...