linux-kernel - Re: [PATCH 6/7] md/raid10, LLVM: get rid of variable length array

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Date:   Fri, 17 Mar 2017 20:27:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Davidson <md@...gle.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Michal Marek <mmarek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Shaohua Li <shli@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>, x86@...nel.org,
        linux-kbuild@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-crypto@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: [PATCH 6/7] md/raid10, LLVM: get rid of variable length array

On Fri, Mar 17, 2017 at 11:52:01AM -0700, Michael Davidson wrote:
> On Fri, Mar 17, 2017 at 5:44 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Be that as it may; what you construct above is disgusting. Surely the
> > code can be refactored to not look like dog vomit?
> >
> > Also; its not immediately obvious conf->copies is 'small' and this
> > doesn't blow up the stack; I feel that deserves a comment somewhere.
> >
> 
> I agree that the code is horrible.
> 
> It is, in fact, exactly the same solution that was used to remove
> variable length arrays in structs from several of the crypto drivers a
> few years ago - see the definition of SHASH_DESC_ON_STACK() in
> "crypto/hash.h" - I did not, however, hide the horrors in a macro
> preferring to leave the implementation visible as a warning to whoever
> might touch the code next.
> 
> I believe that the actual stack usage is exactly the same as it was previously.
> 
> I can certainly wrap this  up in a macro and add comments with
> appropriately dire warnings in it if you feel that is both necessary
> and sufficient.

We got away with ugly in the past, so we should get to do it again?