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

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Date:   Fri, 17 Mar 2017 13:31:23 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michael Davidson <md@...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 1:08 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Mar 16, 2017 at 05:15:19PM -0700, Michael Davidson wrote:
>> Replace a variable length array in a struct by allocating
>> the memory for the entire struct in a char array on the stack.
>>
>> Signed-off-by: Michael Davidson <md@...gle.com>
>> ---
>>  drivers/md/raid10.c | 9 ++++-----
>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>> index 063c43d83b72..158ebdff782c 100644
>> --- a/drivers/md/raid10.c
>> +++ b/drivers/md/raid10.c
>> @@ -4654,11 +4654,10 @@ static int handle_reshape_read_error(struct mddev *mddev,
>>       /* Use sync reads to get the blocks from somewhere else */
>>       int sectors = r10_bio->sectors;
>>       struct r10conf *conf = mddev->private;
>> -     struct {
>> -             struct r10bio r10_bio;
>> -             struct r10dev devs[conf->copies];
>> -     } on_stack;
>> -     struct r10bio *r10b = &on_stack.r10_bio;
>> +     char on_stack_r10_bio[sizeof(struct r10bio) +
>> +                           conf->copies * sizeof(struct r10dev)]
>> +                           __aligned(__alignof__(struct r10bio));
>> +     struct r10bio *r10b = (struct r10bio *)on_stack_r10_bio;
>>       int slot = 0;
>>       int idx = 0;
>>       struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;
>
>
> That's disgusting. Why not fix LLVM to support this?

IIUC there's only a handful of VLAIS instances in LLVM code, why not
just drop them for the sake of better code portability?
(To quote Linus, "this feature is an abomination":
https://lkml.org/lkml/2013/9/23/500)

-- 
Alexander Potapenko
Software Engineer

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