runtime: switch to P 0 before destroying current P
Ps are strictly numbered from 0 to GOMAXPROCS-1, so if procresize
happens to be running on a P that's being destroyed, it moves itself
to P 0.
However, currently procresize destroys the unused Ps *before* moving
itself to P 0. This means it may briefly run on a destroyed P. This is
basically harmless, but has at least one very confusing consequence:
since destroying a P has write barriers, it may enqueue pointers to a
destroyed write barrier buffer. As far as I can tell, there are no
negative consequences of this, but this seems really fragile.
This CL swaps the order of things, so now procresize moves itself to P
0 if necessary before destroying Ps. This ensures it always has a
valid P.
This is part of refactoring for #10958 and #24543, but is a good
cleanup regardless.
Change-Id: I91a23dd6ed27e372f8d6291feec9bc991bcf9812
Reviewed-on: https://go-review.googlesource.com/c/go/+/173941
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>