linux-kernel - Re: [PATCH] objtool,static_call: Don't emit static_call

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Date:   Thu, 18 Mar 2021 09:30:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        Oliver Sang <oliver.sang@...el.com>, jbaron@...mai.com,
        lkp@...ts.01.org, kbuild test robot <lkp@...el.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool,static_call: Don't emit static_call_site for
 .exit.text

On Thu, Mar 18, 2021 at 08:59:45AM +0100, Peter Zijlstra wrote:
> On Wed, Mar 17, 2021 at 07:02:12PM -0500, Josh Poimboeuf wrote:
> > On Wed, Mar 17, 2021 at 01:45:57PM +0100, Peter Zijlstra wrote:
> > > arguably it simply isn't a good idea to use static_call() in __exit
> > > code anyway, since module unload is never a performance critical path.
> > 
> > Couldn't you make the same argument about __init functions, which are
> > allowed to do static calls?
> 
> I suppose we could indeed make that argument. Much of that code was
> copied from jump_label without much consideration. And I now I suppose
> I'll have to consider jump_label in __exit too :/
> 
> > We might consider a STATIC_CALL_SITE_EXIT flag, but I suppose we've run
> > out of flag space.
> 
> Yeah, we're definitely short on flags. Let me try and figure out when
> exactly it's all discarded.

Ha!, x86 stuffs .exit.text in [__init_begin, __init_end) and it is
discarded right along with initmem.

But that means it should match init and be tagged init and all *should*
work, but somehow it doesn't... clearly I'm missing something again
ARGH!