linux-kernel - RE: [RFC 1/7] mac80211: Add check for napi handle before WARN

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Date:   Fri, 24 Jul 2020 11:51:12 +0530
From:   "Rakesh Pillai" <pillair@...eaurora.org>
To:     "'Johannes Berg'" <johannes@...solutions.net>,
        <ath10k@...ts.infradead.org>
Cc:     <linux-wireless@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kvalo@...eaurora.org>, <davem@...emloft.net>, <kuba@...nel.org>,
        <netdev@...r.kernel.org>, <dianders@...omium.org>,
        <evgreen@...omium.org>
Subject: RE: [RFC 1/7] mac80211: Add check for napi handle before WARN_ON



> -----Original Message-----
> From: Johannes Berg <johannes@...solutions.net>
> Sent: Friday, July 24, 2020 1:37 AM
> To: Rakesh Pillai <pillair@...eaurora.org>; ath10k@...ts.infradead.org
> Cc: linux-wireless@...r.kernel.org; linux-kernel@...r.kernel.org;
> kvalo@...eaurora.org; davem@...emloft.net; kuba@...nel.org;
> netdev@...r.kernel.org; dianders@...omium.org; evgreen@...omium.org
> Subject: Re: [RFC 1/7] mac80211: Add check for napi handle before
> WARN_ON
> 
> On Thu, 2020-07-23 at 23:56 +0530, Rakesh Pillai wrote:
> 
> > > > -	WARN_ON_ONCE(softirq_count() == 0);
> > > > +	WARN_ON_ONCE(napi && softirq_count() == 0);
> > >
> > > FWIW, I'm pretty sure this is incorrect - we make assumptions on
> > > softirqs being disabled in mac80211 for serialization and in place of
> > > some locking, I believe.
> > >
> >
> > I checked this, but let me double confirm.
> > But after this change, no packet is submitted from driver in a softirq
> context.
> > So ideally this should take care of serialization.
> 
> I'd guess that we have some reliance on BHs already being disabled, for
> things like u64 sync updates, or whatnot. I mean, we did "rx_ni()" for a
> reason ... Maybe lockdep can help catch some of the issues.
> 
> But couldn't you be in a thread and have BHs disabled too?

This would ideally beat the purpose and possibly hurt the other subsystems running on the same core.

> 
> johannes