2 files changed, 22 insertions, 4 deletions
diff --git a/pkg/sentry/kernel/epoll/epoll.go b/pkg/sentry/kernel/epoll/epoll.go
index 8bffb78fc..592650923 100644
--- a/pkg/sentry/kernel/epoll/epoll.go
+++ b/pkg/sentry/kernel/epoll/epoll.go
@@ -296,8 +296,10 @@ func (*readyCallback) Callback(w *waiter.Entry) {
 		e.waitingList.Remove(entry)
 		e.readyList.PushBack(entry)
 		entry.curList = &e.readyList
+		e.listsMu.Unlock()
 
 		e.Notify(waiter.EventIn)
+		return
 	}
 
 	e.listsMu.Unlock()
diff --git a/pkg/sentry/socket/netstack/netstack.go b/pkg/sentry/socket/netstack/netstack.go
index 13a9a60b4..a2e1da02f 100644
--- a/pkg/sentry/socket/netstack/netstack.go
+++ b/pkg/sentry/socket/netstack/netstack.go
@@ -29,6 +29,7 @@ import (
 	"io"
 	"math"
 	"reflect"
+	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -264,6 +265,12 @@ 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"`
 	// readView contains the remaining payload from the last packet.
@@ -410,21 +417,24 @@ func (s *SocketOperations) isPacketBased() bool {
 
 // fetchReadView updates the readView field of the socket if it's currently
 // empty. It assumes that the socket is locked.
+//
+// Precondition: s.readMu must be held.
 func (s *SocketOperations) fetchReadView() *syserr.Error {
 	if len(s.readView) > 0 {
 		return nil
 	}
-
 	s.readView = nil
 	s.sender = tcpip.FullAddress{}
 
 	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
 }
@@ -623,11 +633,9 @@ 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 {
-		s.readMu.Lock()
-		if len(s.readView) > 0 {
+		if atomic.LoadUint32(&s.readViewHasData) == 1 {
 			r |= waiter.EventIn
 		}
-		s.readMu.Unlock()
 	}
 
 	return r
@@ -2334,6 +2342,10 @@ 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 {
 			err = syserr.FromError(e)
@@ -2456,6 +2468,10 @@ 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