netdev - Re: [PATCH net-next v2 5/5] devlink: Delete reload enable/disable interface

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Date:   Mon, 4 Oct 2021 18:45:07 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Ido Schimmel <idosch@...dia.com>,
        Ingo Molnar <mingo@...hat.com>, Jiri Pirko <jiri@...dia.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        mlxsw@...dia.com, Moshe Shemesh <moshe@...dia.com>,
        netdev@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Salil Mehta <salil.mehta@...wei.com>,
        Shay Drory <shayd@...dia.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Yisen Zhuang <yisen.zhuang@...wei.com>
Subject: Re: [PATCH net-next v2 5/5] devlink: Delete reload enable/disable
 interface

On Mon, Oct 04, 2021 at 05:19:40PM +0300, Ido Schimmel wrote:
> On Sun, Oct 03, 2021 at 09:12:06PM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > After changes to allow dynamically set the reload_up/_down callbacks,
> > we ensure that properly supported devlink ops are not accessible before
> > devlink_register, which is last command in the initialization sequence.
> > 
> > It makes devlink_reload_enable/_disable not relevant anymore and can be
> > safely deleted.
> > 
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> 
> [...]
> 
> > diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
> > index cb6645012a30..09e48fb232a9 100644
> > --- a/drivers/net/netdevsim/dev.c
> > +++ b/drivers/net/netdevsim/dev.c
> > @@ -1512,7 +1512,6 @@ int nsim_dev_probe(struct nsim_bus_dev *nsim_bus_dev)
> >  
> >  	nsim_dev->esw_mode = DEVLINK_ESWITCH_MODE_LEGACY;
> >  	devlink_register(devlink);
> > -	devlink_reload_enable(devlink);
> >  	return 0;
> >  
> >  err_psample_exit:
> > @@ -1566,9 +1565,7 @@ void nsim_dev_remove(struct nsim_bus_dev *nsim_bus_dev)
> >  	struct nsim_dev *nsim_dev = dev_get_drvdata(&nsim_bus_dev->dev);
> >  	struct devlink *devlink = priv_to_devlink(nsim_dev);
> >  
> > -	devlink_reload_disable(devlink);
> >  	devlink_unregister(devlink);
> > -
> >  	nsim_dev_reload_destroy(nsim_dev);
> >  
> >  	nsim_bpf_dev_exit(nsim_dev);
> 
> I didn't remember why devlink_reload_{enable,disable}() were added in
> the first place so it was not clear to me from the commit message why
> they can be removed. It is described in commit a0c76345e3d3 ("devlink:
> disallow reload operation during device cleanup") with a reproducer.

It was added because devlink ops were accessible by the user space very
early in the driver lifetime. All my latest devlink patches are the
attempt to fix this arch/design/implementation issue.

> 
> Tried the reproducer with this series and I cannot reproduce the issue.
> Wasn't quite sure why, but it does not seem to be related to "changes to
> allow dynamically set the reload_up/_down callbacks", as this seems to
> be specific to mlx5.

You didn't reproduce because of my series that moved
devlink_register()/devlink_unregister() to be last/first commands in
.probe()/.remove() flows.

Patch to allow dynamically set ops was needed because mlx5 had logic
like this:
 if(something)
    devlink_reload_enable()

And I needed a way to keep this if ... condition.

> 
> IIUC, the reason that the race described in above mentioned commit can
> no longer happen is related to the fact that devlink_unregister() is
> called first in the device dismantle path, after your previous patches.
> Since both the reload operation and devlink_unregister() hold
> 'devlink_mutex', it is not possible for the reload operation to race
> with device dismantle.
> 
> Agree? If so, I think it would be good to explain this in the commit
> message unless it's clear to everyone else.

I don't agree for very simple reason that devlink_mutex is going to be
removed very soon and it is really not a reason why devlink reload is
safer now when before.

The reload can't race due to:
1. devlink_unregister(), which works as a barrier to stop accesses
from the user space.
2. reference counting that ensures that all in-flight commands are counted.
3. wait_for_completion that blocks till all commands are done.

Thanks

> 
> Thanks