Change SocketOperations.readMu to an RWMutex.

Also get rid of the readViewHasData as it's not required anymore.

Updates #231, #357

PiperOrigin-RevId: 301837227

pkg/sentry/socket/netstack/netstack.go

@@ -29,7 +29,6 @@ import (
 	"io"
 	"math"
 	"reflect"
-	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -265,14 +264,8 @@ type SocketOperations struct {
 	skType   linux.SockType
 	protocol int
 
-	// readViewHasData is 1 iff readView has data to be read, 0 otherwise.
-	// Must be accessed using atomic operations. It must only be written
-	// with readMu held but can be read without holding readMu. The latter
-	// is required to avoid deadlocks in epoll Readiness checks.
-	readViewHasData uint32
-
 	// readMu protects access to the below fields.
-	readMu sync.Mutex `state:"nosave"`
+	readMu sync.RWMutex `state:"nosave"`
 	// readView contains the remaining payload from the last packet.
 	readView buffer.View
 	// readCM holds control message information for the last packet read
@@ -428,13 +421,11 @@ func (s *SocketOperations) fetchReadView() *syserr.Error {
 
 	v, cms, err := s.Endpoint.Read(&s.sender)
 	if err != nil {
-		atomic.StoreUint32(&s.readViewHasData, 0)
 		return syserr.TranslateNetstackError(err)
 	}
 
 	s.readView = v
 	s.readCM = cms
-	atomic.StoreUint32(&s.readViewHasData, 1)
 
 	return nil
 }
@@ -633,9 +624,11 @@ func (s *SocketOperations) Readiness(mask waiter.EventMask) waiter.EventMask {
 	// Check our cached value iff the caller asked for readability and the
 	// endpoint itself is currently not readable.
 	if (mask & ^r & waiter.EventIn) != 0 {
-		if atomic.LoadUint32(&s.readViewHasData) == 1 {
+		s.readMu.RLock()
+		if len(s.readView) > 0 {
 			r |= waiter.EventIn
 		}
+		s.readMu.RUnlock()
 	}
 
 	return r
@@ -2342,9 +2335,6 @@ func (s *SocketOperations) coalescingRead(ctx context.Context, dst usermem.IOSeq
 		}
 		copied += n
 		s.readView.TrimFront(n)
-		if len(s.readView) == 0 {
-			atomic.StoreUint32(&s.readViewHasData, 0)
-		}
 
 		dst = dst.DropFirst(n)
 		if e != nil {
@@ -2468,10 +2458,6 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 		s.readView.TrimFront(int(n))
 	}
 
-	if len(s.readView) == 0 {
-		atomic.StoreUint32(&s.readViewHasData, 0)
-	}
-
 	var flags int
 	if msgLen > int(n) {
 		flags |= linux.MSG_TRUNC