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

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Date:   Thu, 23 Jul 2020 22:06:46 +0200
From:   Johannes Berg <johannes@...solutions.net>
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?

johannes