On Fri, Mar 17, 2017 at 01:31:23PM +0100, Alexander Potapenko wrote:
> On Fri, Mar 17, 2017 at 1:08 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
> >> ---
> >> 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)
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.
