Revert "sync: yield to the waiter when unlocking a starving mutex"

This reverts CL 200577.

Reason for revert: broke linux-arm64-packet and solaris-amd64-oraclerel builders

Fixes #35424
Updates #33747

Change-Id: I2575fd84d37995d458183caae54704f15d8b8426
Reviewed-on: https://go-review.googlesource.com/c/go/+/205817
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

src/runtime/export_test.go

@@ -730,11 +730,3 @@ func RunGetgThreadSwitchTest() {
  panic("g1 != g3")
  }
 }
-
-var Semacquire = semacquire
-var Semrelease1 = semrelease1
-
-func SemNwait(addr *uint32) uint32 {
- root := semroot(addr)
- return atomic.Load(&root.nwait)
-}

src/runtime/proc.go

@@ -2753,22 +2753,7 @@ func preemptPark(gp *g) {
  casGToPreemptScan(gp, _Grunning, _Gscan|_Gpreempted)
  dropg()
  casfrom_Gscanstatus(gp, _Gscan|_Gpreempted, _Gpreempted)
- schedule()
-}
 
-// goyield is like Gosched, but it:
-// - does not emit a GoSched trace event
-// - puts the current G on the runq of the current P instead of the globrunq
-func goyield() {
- checkTimeouts()
- mcall(goyield_m)
-}
-
-func goyield_m(gp *g) {
- pp := gp.m.p.ptr()
- casgstatus(gp, _Grunning, _Grunnable)
- dropg()
- runqput(pp, gp, false)
  schedule()
 }
 

src/runtime/sema.go

@@ -180,7 +180,7 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
  atomic.Xadd(&root.nwait, -1)
  }
  unlock(&root.lock)
- if s != nil { // May be slow or even yield, so unlock first
+ if s != nil { // May be slow, so unlock first
  acquiretime := s.acquiretime
  if acquiretime != 0 {
  mutexevent(t0-acquiretime, 3+skipframes)
@@ -192,25 +192,6 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
  s.ticket = 1
  }
  readyWithTime(s, 5+skipframes)
- if s.ticket == 1 {
- // Direct G handoff
- // readyWithTime has added the waiter G as runnext in the
- // current P; we now call the scheduler so that we start running
- // the waiter G immediately.
- // Note that waiter inherits our time slice: this is desirable
- // to avoid having a highly contended semaphore hog the P
- // indefinitely. goyield is like Gosched, but it does not emit a
- // GoSched trace event and, more importantly, puts the current G
- // on the local runq instead of the global one.
- // We only do this in the starving regime (handoff=true), as in
- // the non-starving case it is possible for a different waiter
- // to acquire the semaphore while we are yielding/scheduling,
- // and this would be wasteful. We wait instead to enter starving
- // regime, and then we start to do direct handoffs of ticket and
- // P.
- // See issue 33747 for discussion.
- goyield()
- }
  }
 }
 

src/sync/mutex.go

@@ -216,8 +216,7 @@ func (m *Mutex) unlockSlow(new int32) {
  old = m.state
  }
  } else {
- // Starving mode: handoff mutex ownership to the next waiter, and yield
- // our time slice so that the next waiter can start to run immediately.
+ // Starving mode: handoff mutex ownership to the next waiter.
  // Note: mutexLocked is not set, the waiter will set it after wakeup.
  // But mutex is still considered locked if mutexStarving is set,
  // so new coming goroutines won't acquire it.