Update to go1.23.4

src/runtime/mgc.go

@@ -190,6 +190,7 @@ func gcinit() {
 	work.markDoneSema = 1
 	lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters)
 	lockInit(&work.assistQueue.lock, lockRankAssistQueue)
+	lockInit(&work.strongFromWeak.lock, lockRankStrongFromWeakQueue)
 	lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
 }
 
@@ -418,6 +419,26 @@ type workType struct {
 		list gList
 	}
 
+	// strongFromWeak controls how the GC interacts with weak->strong
+	// pointer conversions.
+	strongFromWeak struct {
+		// block is a flag set during mark termination that prevents
+		// new weak->strong conversions from executing by blocking the
+		// goroutine and enqueuing it onto q.
+		//
+		// Mutated only by one goroutine at a time in gcMarkDone,
+		// with globally-synchronizing events like forEachP and
+		// stopTheWorld.
+		block bool
+
+		// q is a queue of goroutines that attempted to perform a
+		// weak->strong conversion during mark termination.
+		//
+		// Protected by lock.
+		lock mutex
+		q    gQueue
+	}
+
 	// cycles is the number of completed GC cycles, where a GC
 	// cycle is sweep termination, mark, mark termination, and
 	// sweep. This differs from memstats.numgc, which is
@@ -800,6 +821,19 @@ func gcStart(trigger gcTrigger) {
 // This is protected by markDoneSema.
 var gcMarkDoneFlushed uint32
 
+// gcDebugMarkDone contains fields used to debug/test mark termination.
+var gcDebugMarkDone struct {
+	// spinAfterRaggedBarrier forces gcMarkDone to spin after it executes
+	// the ragged barrier.
+	spinAfterRaggedBarrier atomic.Bool
+
+	// restartedDueTo27993 indicates that we restarted mark termination
+	// due to the bug described in issue #27993.
+	//
+	// Protected by worldsema.
+	restartedDueTo27993 bool
+}
+
 // gcMarkDone transitions the GC from mark to mark termination if all
 // reachable objects have been marked (that is, there are no grey
 // objects and can be no more in the future). Otherwise, it flushes
@@ -842,6 +876,10 @@ top:
 	// stop the world later, so acquire worldsema now.
 	semacquire(&worldsema)
 
+	// Prevent weak->strong conversions from generating additional
+	// GC work. forEachP will guarantee that it is observed globally.
+	work.strongFromWeak.block = true
+
 	// Flush all local buffers and collect flushedWork flags.
 	gcMarkDoneFlushed = 0
 	forEachP(waitReasonGCMarkTermination, func(pp *p) {
@@ -872,6 +910,10 @@ top:
 		goto top
 	}
 
+	// For debugging/testing.
+	for gcDebugMarkDone.spinAfterRaggedBarrier.Load() {
+	}
+
 	// There was no global work, no local work, and no Ps
 	// communicated work since we took markDoneSema. Therefore
 	// there are no grey objects and no more objects can be
@@ -910,6 +952,8 @@ top:
 		}
 	})
 	if restart {
+		gcDebugMarkDone.restartedDueTo27993 = true
+
 		getg().m.preemptoff = ""
 		systemstack(func() {
 			// Accumulate the time we were stopped before we had to start again.
@@ -936,6 +980,11 @@ top:
 	// start the world again.
 	gcWakeAllAssists()
 
+	// Wake all blocked weak->strong conversions. These will run
+	// when we start the world again.
+	work.strongFromWeak.block = false
+	gcWakeAllStrongFromWeak()
+
 	// Likewise, release the transition lock. Blocked
 	// workers and assists will run when we start the
 	// world again.