netdev - Re: [PATCH wpan-next v2 10/11] net: mac802154: Add a warning in the hot path

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Date:   Wed, 18 May 2022 11:13:26 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Alexander Aring <aahringo@...hat.com>
Cc:     Alexander Aring <alex.aring@...il.com>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH wpan-next v2 10/11] net: mac802154: Add a warning in the
 hot path

Hi Alex,

aahringo@...hat.com wrote on Tue, 17 May 2022 20:59:39 -0400:

> Hi,
> 
> On Tue, May 17, 2022 at 10:53 AM Miquel Raynal
> <miquel.raynal@...tlin.com> wrote:
> >
> >
> > miquel.raynal@...tlin.com wrote on Tue, 17 May 2022 15:36:55 +0200:
> >  
> > > aahringo@...hat.com wrote on Sun, 15 May 2022 18:30:15 -0400:
> > >  
> > > > Hi,
> > > >
> > > > On Thu, May 12, 2022 at 10:34 AM Miquel Raynal
> > > > <miquel.raynal@...tlin.com> wrote:  
> > > > >
> > > > > We should never start a transmission after the queue has been stopped.
> > > > >
> > > > > But because it might work we don't kill the function here but rather
> > > > > warn loudly the user that something is wrong.
> > > > >
> > > > > Set an atomic when the queue will remain stopped. Reset this atomic when
> > > > > the queue actually gets restarded. Just check this atomic to know if the
> > > > > transmission is legitimate, warn if it is not.
> > > > >
> > > > > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> > > > > ---
> > > > >  include/net/cfg802154.h |  1 +
> > > > >  net/mac802154/tx.c      | 16 +++++++++++++++-
> > > > >  net/mac802154/util.c    |  1 +
> > > > >  3 files changed, 17 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/net/cfg802154.h b/include/net/cfg802154.h
> > > > > index 8b6326aa2d42..a1370e87233e 100644
> > > > > --- a/include/net/cfg802154.h
> > > > > +++ b/include/net/cfg802154.h
> > > > > @@ -218,6 +218,7 @@ struct wpan_phy {
> > > > >         struct mutex queue_lock;
> > > > >         atomic_t ongoing_txs;
> > > > >         atomic_t hold_txs;
> > > > > +       atomic_t queue_stopped;  
> > > >
> > > > Maybe some test_bit()/set_bit() is better there?  
> > >
> > > What do you mean? Shall I change the atomic_t type of queue_stopped?
> > > Isn't the atomic_t preferred in this situation?  
> >
> > Actually I re-read the doc and that's right, a regular unsigned long  
> 
> Which doc is that?

Documentation/atomic_t.txt states [SEMANTICS chapter]:

	"if you find yourself only using the Non-RMW operations of
	atomic_t, you do not in fact need atomic_t at all and are doing it wrong."

In this case, I was only using atomic_set() and atomic_read(), which are
both non-RMW operations.

Thanks,
Miquèl