netdev - Re: [PATCH net-next 1/3] net: provide macros for commonly copied lockless queue stop/wake code

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Message-ID: <c3b05efb-e691-4947-84f9-cf524e7d2cd9@paulmck-laptop>
Date:   Thu, 6 Apr 2023 08:45:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com
Subject: Re: [PATCH net-next 1/3] net: provide macros for commonly copied
 lockless queue stop/wake code

On Thu, Apr 06, 2023 at 07:46:48AM -0700, Jakub Kicinski wrote:
> On Thu, 6 Apr 2023 07:17:09 -0700 Paul E. McKenney wrote:
> > > > Mightn't preemption or interrupts cause further issues?  Or are preemption
> > > > and/or interrupts disabled across the relevant sections of code?  
> > > 
> > > The code in question is supposed to run in softirq context.  So
> > > both interrupts and preemption should be disabled.  
> > 
> > Agreed, preemption will be enabled in softirq, but interrupts can still
> > happen, correct?
> 
> Starting the queue only happens from softirq (I hope) and stopping 
> can happen from any context. So we're risking false-starts again.
> I think this puts to bed any hope of making this code safe against
> false-starts with just barriers :(

Is it possible to jam all the relevant state into a single variable?
(I believe that that answer is "no", but just in case asking this question
inspires someone to come up with a good idea.)

							Thanx, Paul