virtio-devices: Make pause idempotent to prevent deadlock

Previously, calling pause() when already paused would wait on a barrier
for worker threads that were already parked, causing a deadlock.

This situation occurs when the VMM thread holds a device mutex while
calling an operation that triggers pause(), and a vCPU thread
simultaneously needs that same mutex for MMIO access. With slow I/O
backends (like RBD/Ceph), the timing window for this race is larger,
making the deadlock more likely to occur, see [0].

Make pause() idempotent by checking the paused state atomically and
returning early if already paused, avoiding the barrier wait.

[0] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/7948#discussion_r305052509

Signed-off-by: Vincent Thomas <vincent@v-thomas.com>

virtio-devices/src/device.rs

@@ -340,7 +340,13 @@ impl Pausable for VirtioCommon {
             "Pausing virtio-{}",
             VirtioDeviceType::from(self.device_type)
         );
-        self.paused.store(true, Ordering::SeqCst);
+
+        // If already paused, return early to avoid deadlock waiting on barrier
+        // for worker threads that are already parked.
+        if self.paused.swap(true, Ordering::SeqCst) {
+            return Ok(());
+        }
+
         if let Some(pause_evt) = &self.pause_evt {
             pause_evt
                 .write(1)