systrap: remove the subprocess list

It was used to disable the fast path in stub threads.  The current
implemenation has a race when subprocessList.PushBack(s) can
be called before subprocessList.Remove(s). This issue can be fixed
by introducing a new mutex to guard contextQueue.numAwakeContexts,
but it will introduce additional overhead.

It was an addition way to disable the stub fast path. We do the
same thing when a new context is added to a subprocess context queue.

PiperOrigin-RevId: 533297618

pkg/sentry/platform/systrap/BUILD

@@ -18,18 +18,6 @@ go_template_instance(
     },
 )
 
-go_template_instance(
-    name = "subprocess_list",
-    out = "subprocess_list.go",
-    package = "systrap",
-    prefix = "subprocess",
-    template = "//pkg/ilist:generic_list",
-    types = {
-        "Element": "*subprocess",
-        "Linker": "*subprocess",
-    },
-)
-
 go_template_instance(
     name = "subprocess_refs",
     out = "subprocess_refs.go",
@@ -64,7 +52,6 @@ go_library(
         "subprocess_arm64.go",
         "subprocess_linux.go",
         "subprocess_linux_unsafe.go",
-        "subprocess_list.go",
         "subprocess_pool.go",
         "subprocess_refs.go",
         "subprocess_unsafe.go",

pkg/sentry/platform/systrap/shared_context.go

@@ -210,11 +210,6 @@ type fastPathDispatcher struct {
 	// fastPathDisabledTS is the time stamp when the stub fast path was
 	// disabled. It is zero if the fast path is enabled.
 	fastPathDisabledTS atomic.Uint64
-
-	subprocessListMu sync.Mutex
-	// subprocessList contains subprocesses with at least one awake context.
-	// +checklocks:subprocessListMu
-	subprocessList subprocessList
 }
 
 var dispatcher fastPathDispatcher
@@ -256,29 +251,9 @@ func (q *fastPathDispatcher) stubFastPathEnabled() bool {
 // disableStubFastPath disables the fast path over all subprocesses with active
 // contexts.
 func (q *fastPathDispatcher) disableStubFastPath() {
-	q.subprocessListMu.Lock()
-	defer q.subprocessListMu.Unlock()
-
-	for s := q.subprocessList.Front(); s != nil; s = s.Next() {
-		s.contextQueue.disableFastPath()
-	}
 	q.fastPathDisabledTS.Store(uint64(cputicks()))
 }
 
-func (q *fastPathDispatcher) activateSubprocess(s *subprocess) {
-	q.subprocessListMu.Lock()
-	defer q.subprocessListMu.Unlock()
-
-	q.subprocessList.PushBack(s)
-}
-
-func (q *fastPathDispatcher) deactivateSubprocess(s *subprocess) {
-	q.subprocessListMu.Lock()
-	defer q.subprocessListMu.Unlock()
-
-	q.subprocessList.Remove(s)
-}
-
 // deep_sleep_timeout is the timeout after which we stops polling and fall asleep.
 //
 // The value is 40µs for 2GHz CPU. This timeout matches the sentry<->stub round

pkg/sentry/platform/systrap/subprocess.go

@@ -121,7 +121,6 @@ const (
 type subprocess struct {
 	platform.NoAddressSpaceIO
 	subprocessRefs
-	subprocessEntry
 
 	// requests is used to signal creation of new threads.
 	requests chan any
@@ -689,14 +688,9 @@ func (s *subprocess) incAwakeContexts() {
 	if nr > uint32(maxSysmsgThreads) {
 		return
 	}
-	if nr == 1 {
-		dispatcher.activateSubprocess(s)
-	}
 	nr = nrMaxAwakeStubThreads.Add(1)
 	if nr > fastPathContextLimit {
-		if dispatcher.stubFastPathEnabled() {
-			dispatcher.disableStubFastPath()
-		}
+		dispatcher.disableStubFastPath()
 	}
 }
 
@@ -705,9 +699,6 @@ func (s *subprocess) decAwakeContexts() {
 	if nr >= uint32(maxSysmsgThreads) {
 		return
 	}
-	if nr == 0 {
-		dispatcher.deactivateSubprocess(s)
-	}
 	nrMaxAwakeStubThreads.Add(^uint32(0))
 }