Excerpts from Andy Lutomirski's message of July 14, 2020 1:48 am:

On Mon, Jul 13, 2020 at 7:13 AM Mathieu Desnoyers
[off-list ref] wrote:

quoted

----- On Jul 13, 2020, at 9:47 AM, Nicholas Piggin npiggin@gmail.com wrote:

quoted

Excerpts from Nicholas Piggin's message of July 13, 2020 2:45 pm:

quoted

Excerpts from Andy Lutomirski's message of July 11, 2020 3:04 am:

quoted

Also, as it stands, I can easily see in_irq() ceasing to promise to
serialize.  There are older kernels for which it does not promise to
serialize.  And I have plans to make it stop serializing in the
nearish future.

You mean x86's return from interrupt? Sounds fun... you'll konw where to
update the membarrier sync code, at least :)

Oh, I should actually say Mathieu recently clarified a return from
interrupt doesn't fundamentally need to serialize in order to support
membarrier sync core.

Clarification to your statement:

Return from interrupt to kernel code does not need to be context serializing
as long as kernel serializes before returning to user-space.

However, return from interrupt to user-space needs to be context serializing.

Indeed, and I figured this out on the first read through because I'm
quite familiar with the x86 entry code.  But Nick somehow missed this,
and Nick is the one who wrote the patch.

Nick, I think this helps prove my point.  The code you're submitting
may well be correct, but it's unmaintainable.

It's not. The patch I wrote for x86 is a no-op, it just moves existing
x86 hook and code that's already there to a different name.

Actually it's not quite a no-op, it't changes it to use hooks that are
actually called in the right places. Because previously it was
unmaintainable from point of view of generic mm -- it was not clear at
all that the old one should have been called in other places where the
mm goes non-lazy. Now with the exit_lazy_tlb hook, it can quite easily
be spotted where it is missing.

And x86 keeps their membarrier code in x86, and uses nice well defined
lazy tlb mm hooks.

At the very least, this
needs a comment explaining, from the perspective of x86, *exactly*
what exit_lazy_tlb() is promising, why it's promising it, how it
achieves that promise, and what code cares about it.  Or we could do
something with TIF flags and make this all less magical, although that
will probably end up very slightly slower.

It's all documented there in existing comments plus the asm-generic
exit_lazy_tlb specification added AFAIKS.

Is the membarrier comment in finish_task_switch plus these ones not
enough?

Thanks,
Nick