diff --git a/src/main/java/org/jenkinsci/plugins/workflow/log/FileLogStorage.java b/src/main/java/org/jenkinsci/plugins/workflow/log/FileLogStorage.java
index a4d7ae9a..0c1b750a 100644
--- a/src/main/java/org/jenkinsci/plugins/workflow/log/FileLogStorage.java
+++ b/src/main/java/org/jenkinsci/plugins/workflow/log/FileLogStorage.java
@@ -26,6 +26,7 @@
 
 import edu.umd.cs.findbugs.annotations.NonNull;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+import hudson.Functions;
 import hudson.console.AnnotatedLargeText;
 import hudson.console.ConsoleAnnotationOutputStream;
 import hudson.model.BuildListener;
@@ -94,6 +95,14 @@ private FileLogStorage(File log) {
     private synchronized void open() throws IOException {
         if (os == null) {
             os = new FileOutputStream(log, true);
+            // TODO mandatory file locks break log reading on Windows (see FileLogStorageTest.smokes etc.)
+            // And LargeText offers no control over how FileSession works, so cannot share the channel.
+            // Could instead lock the index file, then use openStorages to ensure that readers reuse the channel.
+            if (!Functions.isWindows()) {
+                LOGGER.fine(() -> "locking " + log + "…");
+                os.getChannel().lock();
+                LOGGER.fine(() -> "…locked " + log);
+            }
             osStartPosition = log.length();
             cos = new CountingOutputStream(os);
             bos = LogStorage.wrapWithAutoFlushingBuffer(cos);
@@ -189,6 +198,9 @@ private final class IndexOutputStream extends OutputStream {
                 openStorages.remove(log);
                 try {
                     bos.close();
+                    if (!Functions.isWindows()) {
+                        LOGGER.fine(() -> "closed " + log + " which should have released its lock");
+                    }
                 } finally {
                     indexOs.close();
                 }
diff --git a/src/test/java/org/jenkinsci/plugins/workflow/log/FileLogStorageTest.java b/src/test/java/org/jenkinsci/plugins/workflow/log/FileLogStorageTest.java
index fb35e8fe..b96011eb 100644
--- a/src/test/java/org/jenkinsci/plugins/workflow/log/FileLogStorageTest.java
+++ b/src/test/java/org/jenkinsci/plugins/workflow/log/FileLogStorageTest.java
@@ -28,13 +28,17 @@
 
 import hudson.model.TaskListener;
 import java.io.File;
+import java.util.logging.Level;
 import org.junit.Before;
+import org.junit.ClassRule;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
+import org.jvnet.hudson.test.LoggerRule;
 
 public class FileLogStorageTest extends LogStorageTestBase {
 
+    @ClassRule public static LoggerRule fineLogging = new LoggerRule();
     @Rule public TemporaryFolder tmp = new TemporaryFolder();
     private File log;
 
@@ -56,6 +60,7 @@ public class FileLogStorageTest extends LogStorageTestBase {
     }
 
     @Test public void interruptionDoesNotCloseStream() throws Exception {
+        fineLogging.record(FileLogStorage.class, Level.FINE);
         LogStorage ls = createStorage();
         TaskListener overall = ls.overallListener();
         overall.getLogger().println("overall 1");