runtime: prepare for extensions to waiting M list

Move the nextwaitm field into a small struct, in preparation for
additional metadata to track how long Ms need to wait for locks.

For #66999

Change-Id: Ib40e43c15cde22f7e35922641107973d99439ecd
Reviewed-on: https://go-review.googlesource.com/c/go/+/585635
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/runtime/mprof.go b/src/runtime/mprof.go
index b51a1ad3ce..b97fac787e 100644
--- a/src/runtime/mprof.go
+++ b/src/runtime/mprof.go
@@ -667,6 +667,17 @@ func (lt *lockTimer) end() {
 	}
 }
 
+// mWaitList is part of the M struct, and holds the list of Ms that are waiting
+// for a particular runtime.mutex.
+//
+// When an M is unable to immediately obtain a lock, it adds itself to the list
+// of Ms waiting for the lock. It does that via this struct's next field,
+// forming a singly-linked list with the mutex's key field pointing to the head
+// of the list.
+type mWaitList struct {
+	next muintptr // next m waiting for lock (set by us, cleared by another during unlock)
+}
+
 type mLockProfile struct {
 	waitTime   atomic.Int64 // total nanoseconds spent waiting in runtime.lockWithRank
 	stack      []uintptr    // stack that experienced contention in runtime.lockWithRank