runtime: ensure mheap lock stack growth invariant is maintained

Currently there's an invariant in the runtime wherein the heap lock
can only be acquired on the system stack, otherwise a self-deadlock
could occur if the stack grows while the lock is held.

This invariant is upheld and documented in a number of situations (e.g.
allocManual, freeManual) but there are other places where the invariant
is either not maintained at all which risks self-deadlock (e.g.
setGCPercent, gcResetMarkState, allocmcache) or is maintained but
undocumented (e.g. gcSweep, readGCStats_m).

This change adds go:systemstack to any function that acquires the heap
lock or adds a systemstack(func() { ... }) around the critical section,
where appropriate. It also documents the invariant on (*mheap).lock
directly and updates repetitive documentation to refer to that comment.

Fixes #32105.

Change-Id: I702b1290709c118b837389c78efde25c51a2cafb
Reviewed-on: https://go-review.googlesource.com/c/go/+/177857
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/runtime/mstats.go b/src/runtime/mstats.go
index 9250865ed1..421580eec3 100644
--- a/src/runtime/mstats.go
+++ b/src/runtime/mstats.go
@@ -470,6 +470,9 @@ func readGCStats(pauses *[]uint64) {
 	})
 }
 
+// readGCStats_m must be called on the system stack because it acquires the heap
+// lock. See mheap for details.
+//go:systemstack
 func readGCStats_m(pauses *[]uint64) {
 	p := *pauses
 	// Calling code in runtime/debug should make the slice large enough.