[syzbot] [arm?] WARNING in do_sve_acc

[Mark Brown]

On Wed, Jan 03, 2024 at 06:05:49PM +0000, Will Deacon wrote:
> [+Broonie and Mark R as I'm sure we've seen this one before, possibly
>  in a report from Naresh? Was it a QEMU issue?]

It's been reported by the LKFT people, but it occurs with *extremely*
low frequency, there's no reproducer and nothing showed up from code
inspection by myself and Mark.  The reports suggest it's been there
since ~v5.15 (that's the oldest stable it was seen in and Linaro do
still cover older), though Linaro's reports were conflating multiple
potentially related symptoms so it's a little unclear what exactly was
seen in what version.

I suspect to the extent it's qemu it's just that that's mostly where
people are exercising SVE.  This was shown with a userspace with SVE
usage in glibc, I *think* that upgrading to a similar glibc version was
what started Linaro seeing things too based on the release dates for the
particular stable versions they mentioned but didn't get them to confirm
yet.

I *might've* been able to show something with current code just before
Christmas but the symptoms are a bit different, I was going to start
looking when I get back.

> > Unfortunately, I don't have any reproducer for this issue yet.

Indeed...

> > compiler:       aarch64-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

> > pc : do_sve_acc+0x6c/0x10c arch/arm64/kernel/fpsimd.c:1394
> > lr : do_sve_acc+0x5c/0x10c arch/arm64/kernel/fpsimd.c:1391
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