Don't acquire contended lock with the OS thread locked.

Fixes #1049

PiperOrigin-RevId: 297175164

1 files changed, 17 insertions, 1 deletions

diff --git a/pkg/sentry/platform/kvm/machine.go b/pkg/sentry/platform/kvm/machine.go
index 8076c7529..f1afc74dc 100644
--- a/pkg/sentry/platform/kvm/machine.go
+++ b/pkg/sentry/platform/kvm/machine.go
@@ -329,10 +329,12 @@ func (m *machine) Destroy() {
 }
 
 // Get gets an available vCPU.
+//
+// This will return with the OS thread locked.
 func (m *machine) Get() *vCPU {
+	m.mu.RLock()
 	runtime.LockOSThread()
 	tid := procid.Current()
-	m.mu.RLock()
 
 	// Check for an exact match.
 	if c := m.vCPUs[tid]; c != nil {
@@ -343,8 +345,22 @@ func (m *machine) Get() *vCPU {
 
 	// The happy path failed. We now proceed to acquire an exclusive lock
 	// (because the vCPU map may change), and scan all available vCPUs.
+	// In this case, we first unlock the OS thread. Otherwise, if mu is
+	// not available, the current system thread will be parked and a new
+	// system thread spawned. We avoid this situation by simply refreshing
+	// tid after relocking the system thread.
 	m.mu.RUnlock()
+	runtime.UnlockOSThread()
 	m.mu.Lock()
+	runtime.LockOSThread()
+	tid = procid.Current()
+
+	// Recheck for an exact match.
+	if c := m.vCPUs[tid]; c != nil {
+		c.lock()
+		m.mu.Unlock()
+		return c
+	}
 
 	for {
 		// Scan for an available vCPU.