linux-kernel - Re: [PATCH v2 1/2] livepatch: Remove duplicate warning about missing reliable stacktrace support

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Date:   Thu, 9 May 2019 09:24:53 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] livepatch: Remove duplicate warning about missing
 reliable stacktrace support


> > > But I think Miroslav's suggestion to revert 1d98a69e5cef would be even
> > > better.
> > 
> > AFAIK, Miroslav wanted to point out that your opinion was inconsistent.
> 
> How is my opinion inconsistent?
> 
> Is there something wrong with the original approach, which was reverted
> with
> 
>   1d98a69e5cef ("livepatch: Remove reliable stacktrace check in klp_try_switch_task()")
> 
> ?
> 
> As I mentioned, that has some advantages:
> 
> - The generated code is simpler/faster because it uses a build-time
>   check.
> 
> - The code is more readable IMO.  Especially if the check is done higher
>   up the call stack by reverting 1d98a69e5cef.  That way the arch
>   support short-circuiting is done in the logical place, before doing
>   any more unnecessary work.  It's faster, but also, more importantly,
>   it's less surprising.

Correct. I forgot we removed return from klp_enable_patch() if 
klp_have_reliable_stack() errors out and we only warn now. So reverting 
1d98a69e5cef definitely makes sense.

My...

"We removed it in 1d98a69e5cef ("livepatch: Remove reliable stacktrace 
check in klp_try_switch_task()") and I do think it does not belong here. 
We can check for the facility right at the beginning in klp_enable_patch() 
and it is not necessary to do it every time klp_check_stack() is called." 

...from the other email is rubbish then.

Miroslav