runtime: ensure heap memstats are updated atomically

For the most part, heap memstats are already updated atomically when
passed down to OS-level memory functions (e.g. sysMap). Elsewhere,
however, they're updated with the heap lock.

In order to facilitate holding the heap lock for less time during
allocation paths, this change more consistently makes the update of
these statistics atomic by calling mSysStat{Inc,Dec} appropriately
instead of simply adding or subtracting. It also ensures these values
are loaded atomically.

Furthermore, an undocumented but safe update condition for these
memstats is during STW, at which point using atomics is unnecessary.
This change also documents this condition in mstats.go.

Updates #35112.

Change-Id: I87d0b6c27b98c88099acd2563ea23f8da1239b66
Reviewed-on: https://go-review.googlesource.com/c/go/+/196638
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
3 files changed
tree: 3052e5f9bb07210295a69ea8eebfe6be4cb141b8
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
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