linux-kernel - Re: [PATCH RFC V3 6/9] x86/entry: Pass irqentry_state

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Date:   Tue, 20 Oct 2020 07:10:29 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC V3 6/9] x86/entry: Pass irqentry_state_t by reference

On Mon, Oct 19, 2020 at 11:12:44PM +0200, Thomas Gleixner wrote:
> On Mon, Oct 19 2020 at 13:26, Ira Weiny wrote:
> > On Mon, Oct 19, 2020 at 11:32:50AM +0200, Thomas Gleixner wrote:
> > Sorry, let me clarify.  After this patch we have.
> >
> > typedef union irqentry_state {
> > 	bool	exit_rcu;
> > 	bool	lockdep;
> > } irqentry_state_t;
> >
> > Which reflects the mutual exclusion of the 2 variables.
> 
> Huch? From the patch I gave you:
> 
>  #ifndef irqentry_state
>  typedef struct irqentry_state {
>  	bool    exit_rcu;
> +       bool    lockdep;
>  } irqentry_state_t;
>  #endif
> 
> How is that a union?

I was proposing to make it a union.

> 
> > But then when the pkrs stuff is added the union changes back to a structure and
> > looks like this.
> 
> So you want:
> 
>   1) Move stuff to struct irqentry_state (my patch)
> 
>   2) Change it to a union and pass it as pointer at the same time

No, I would have made it a union in your patch.

Pass by reference would remain largely the same.

> 
>   3) Change it back to struct to add PKRS

Yes.  :-/

> 
> > Is that clear?
> 
> What's clear is that the above is nonsense. We can just do
> 
>  #ifndef irqentry_state
>  typedef struct irqentry_state {
>  	union {
>          	bool    exit_rcu;
>                 bool    lockdep;
>         };        
>  } irqentry_state_t;
>  #endif
> 
> right in the patch which I gave you. Because that actually makes sense.

Ok I'm very sorry.  I was thinking that having a struct containing nothing but
an anonymous union would be unacceptable as a stand alone item in your patch.
In my experience other maintainers would have rejected such a change and
would have asked; 'why not just make it a union'?

I'm very happy skipping the gymnastics on individual patches in favor of making
the whole series work out in the end.

Thank you for your help again.  :-)

Ira