On Wed, 13 Jan 2021 at 23:09, Linus Torvalds
[off-list ref] wrote:

On Wed, Jan 13, 2021 at 1:44 PM Russell King - ARM Linux admin
[off-list ref] wrote:

quoted

So, maybe the Sparc issue was just a similar but different bug in gcc
4.9.x.

Good catch. And I know this bug has happened independently on
different architectures several times (I remember this on x86-64 as
well), so I started looking around.

And in fact, 4.9 was buggy on x86-64 too:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61904

And yeah, _that_ gcc bug wasn't actually x86-64 specific, but
apparently a generic instruction scheduling bug.

So it's an independent bug, but I do have to admit that the arguments
against 4.9 are piling up (even if that particular fix apparently got
fixed in the gcc branches and apparently backported to distro
compilers too).

So if the arguments are piling up, what is holding us back, other than
inertia? RHEL 7 used to be a factor, but it ships with 4.8 not 4.9, so
its users already need to upgrade. Is anyone aware of a good reason to
keep 4.9 supported? Are any other long term supported distros using
4.9.x?

I know that distros probably backported fixes for all of these issues,
but without a way to interrogate the compiler about this, that doesn't
really make a difference IMHO.

Note that banning 4.9 for arm64 and banning it in general should be
two different changes in any case, as the former will need to be
backported to -stable kernels as well.

 --
Ard.

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