diff --git a/src/Cache.zig b/src/Cache.zig
index 829d903e5a..da1e056644 100644
--- a/src/Cache.zig
+++ b/src/Cache.zig
@@ -690,7 +690,7 @@ pub const Manifest = struct {
         while (true) {
             switch (it.next() orelse return) {
                 .target, .target_must_resolve => return,
-                .prereq => |bytes| try self.addFilePost(bytes),
+                .prereq => |file_path| try self.addFilePost(file_path),
                 else => |err| {
                     try err.printError(error_buf.writer());
                     log.err("failed parsing {s}: {s}", .{ dep_file_basename, error_buf.items });
diff --git a/src/Compilation.zig b/src/Compilation.zig
index 2e8ba98a89..958aac5e1b 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -48,6 +48,7 @@ bin_file: *link.File,
 c_object_table: std.AutoArrayHashMapUnmanaged(*CObject, void) = .{},
 /// This is a pointer to a local variable inside `update()`.
 whole_cache_manifest: ?*Cache.Manifest = null,
+whole_cache_manifest_mutex: std.Thread.Mutex = .{},
 
 link_error_flags: link.File.ErrorFlags = .{},
 
@@ -2199,8 +2200,8 @@ pub fn update(comp: *Compilation) !void {
         // We are about to obtain this lock, so here we give other processes a chance first.
         comp.bin_file.releaseLock();
 
-        comp.whole_cache_manifest = &man;
         man = comp.cache_parent.obtain();
+        comp.whole_cache_manifest = &man;
         try comp.addNonIncrementalStuffToCacheManifest(&man);
 
         const is_hit = man.hit() catch |err| {
@@ -3598,6 +3599,8 @@ pub fn cImport(comp: *Compilation, c_src: []const u8) !CImportResult {
         const dep_basename = std.fs.path.basename(out_dep_path);
         try man.addDepFilePost(zig_cache_tmp_dir, dep_basename);
         if (comp.whole_cache_manifest) |whole_cache_manifest| {
+            comp.whole_cache_manifest_mutex.lock();
+            defer comp.whole_cache_manifest_mutex.unlock();
             try whole_cache_manifest.addDepFilePost(zig_cache_tmp_dir, dep_basename);
         }
 
@@ -4060,6 +4063,11 @@ fn updateCObject(comp: *Compilation, c_object: *CObject, c_obj_prog_node: *std.P
             const dep_basename = std.fs.path.basename(dep_file_path);
             // Add the files depended on to the cache system.
             try man.addDepFilePost(zig_cache_tmp_dir, dep_basename);
+            if (comp.whole_cache_manifest) |whole_cache_manifest| {
+                comp.whole_cache_manifest_mutex.lock();
+                defer comp.whole_cache_manifest_mutex.unlock();
+                try whole_cache_manifest.addDepFilePost(zig_cache_tmp_dir, dep_basename);
+            }
             // Just to save disk space, we delete the file because it is never needed again.
             zig_cache_tmp_dir.deleteFile(dep_basename) catch |err| {
                 log.warn("failed to delete '{s}': {s}", .{ dep_file_path, @errorName(err) });
diff --git a/src/Module.zig b/src/Module.zig
index a8ea63ffc9..2bda1707b2 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -4523,7 +4523,7 @@ pub fn semaFile(mod: *Module, file: *File) SemaError!void {
             error.AnalysisFail => {},
         }
 
-        if (mod.comp.whole_cache_manifest) |man| {
+        if (mod.comp.whole_cache_manifest) |whole_cache_manifest| {
             const source = file.getSource(gpa) catch |err| {
                 try reportRetryableFileError(mod, file, "unable to load source: {s}", .{@errorName(err)});
                 return error.AnalysisFail;
@@ -4541,7 +4541,9 @@ pub fn semaFile(mod: *Module, file: *File) SemaError!void {
             };
             errdefer gpa.free(resolved_path);
 
-            try man.addFilePostContents(resolved_path, source.bytes, source.stat);
+            mod.comp.whole_cache_manifest_mutex.lock();
+            defer mod.comp.whole_cache_manifest_mutex.unlock();
+            try whole_cache_manifest.addFilePostContents(resolved_path, source.bytes, source.stat);
         }
     } else {
         new_decl.analysis = .file_failure;
@@ -5036,10 +5038,12 @@ pub fn embedFile(mod: *Module, cur_file: *File, rel_file_path: []const u8) !*Emb
         resolved_root_path, resolved_path, sub_file_path, rel_file_path,
     });
 
-    if (mod.comp.whole_cache_manifest) |man| {
+    if (mod.comp.whole_cache_manifest) |whole_cache_manifest| {
         const copied_resolved_path = try gpa.dupe(u8, resolved_path);
         errdefer gpa.free(copied_resolved_path);
-        try man.addFilePostContents(copied_resolved_path, bytes, stat);
+        mod.comp.whole_cache_manifest_mutex.lock();
+        defer mod.comp.whole_cache_manifest_mutex.unlock();
+        try whole_cache_manifest.addFilePostContents(copied_resolved_path, bytes, stat);
     }
 
     keep_resolved_path = true; // It's now owned by embed_table.
diff --git a/src/stage1.zig b/src/stage1.zig
index f4c9cbf8a1..bb2752c802 100644
--- a/src/stage1.zig
+++ b/src/stage1.zig
@@ -459,10 +459,11 @@ export fn stage2_fetch_file(
     const comp = @intToPtr(*Compilation, stage1.userdata);
     const file_path = path_ptr[0..path_len];
     const max_file_size = std.math.maxInt(u32);
-    const contents = if (comp.whole_cache_manifest) |man|
-        man.addFilePostFetch(file_path, max_file_size) catch return null
-    else
-        std.fs.cwd().readFileAlloc(comp.gpa, file_path, max_file_size) catch return null;
+    const contents = if (comp.whole_cache_manifest) |man| blk: {
+        comp.whole_cache_manifest_mutex.lock();
+        defer comp.whole_cache_manifest_mutex.unlock();
+        break :blk man.addFilePostFetch(file_path, max_file_size) catch return null;
+    } else std.fs.cwd().readFileAlloc(comp.gpa, file_path, max_file_size) catch return null;
     result_len.* = contents.len;
     // TODO https://github.com/ziglang/zig/issues/3328#issuecomment-716749475
     if (contents.len == 0) return @intToPtr(?[*]const u8, 0x1);