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

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Date:   Fri, 17 Mar 2017 13:08:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Davidson <md@...gle.com>
Cc:     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>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>, x86@...nel.org,
        linux-kbuild@...r.kernel.org, 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 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?