netdev - RE: [PATCH net-next] net: sch: prio: Set bands to default on delete instead of noop

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Message-ID: <AM0PR05MB47211C315F8CD6C95069A46EAC830@AM0PR05MB4721.eurprd05.prod.outlook.com>
Date:   Sun, 29 Apr 2018 08:13:28 +0000
From:   Nogah Frankel <nogahf@...lanox.com>
To:     David Miller <davem@...emloft.net>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "jhs@...atatu.com" <jhs@...atatu.com>, mlxsw <mlxsw@...lanox.com>
Subject: RE: [PATCH net-next] net: sch: prio: Set bands to default on delete
 instead of noop

> > When a band is created, it is set to the default qdisc, which is
> > "invisible" pfifo.
> > However, if a band is set to a qdisc that is later being deleted, it will
> > be set to noop qdisc. This can cause a packet loss, while there is no clear
> > user indication for it. ("invisible" qdisc are not being shown by default).
> > This patch sets a band to the default qdisc, rather then the noop qdisc, on
> > delete operation.
> >
> > Signed-off-by: Nogah Frankel <nogahf@...lanox.com>
> 
> Like Cong, I'm worried this will break something.  The code has
> behaved this way for 2 decades or longer.
> 
> If you want to put another qdisc there, and thus not drop any traffic,
> modify the qdisc to a new one instead of performing a delete operation.

I believe that this historic behavior is quite bug like, especially because
TCA_DUMP_INVISIBLE is not set by default.
Yet I accept your judgment that changing it now might break things.
Thank you.
 
Nogah