Fix deadlock between epoll_wait and getdents

epoll_wait acquires EventPoll.listsMu (in EventPoll.ReadEvents) and
then calls Inotify.Readiness which tries to acquire Inotify.evMu.

getdents acquires Inotify.evMu (in Inotify.queueEvent) and then calls
readyCallback.Callback which tries to acquire EventPoll.listsMu.

The fix is to release Inotify.evMu before calling Queue.Notify. Queue
is thread-safe and doesn't require Inotify.evMu to be held.

Closes #121

PiperOrigin-RevId: 227066695
Change-Id: Id29364bb940d1727f33a5dff9a3c52f390c15761

1 files changed, 55 insertions, 0 deletions

diff --git a/test/syscalls/linux/inotify.cc b/test/syscalls/linux/inotify.cc
index 0e361496c..167ca44a8 100644
--- a/test/syscalls/linux/inotify.cc
+++ b/test/syscalls/linux/inotify.cc
@@ -14,9 +14,12 @@
 
 #include <fcntl.h>
 #include <libgen.h>
+#include <sched.h>
+#include <sys/epoll.h>
 #include <sys/inotify.h>
 #include <sys/ioctl.h>
 
+#include <atomic>
 #include <list>
 #include <string>
 #include <vector>
@@ -24,6 +27,9 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/str_join.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+#include "test/util/epoll_util.h"
 #include "test/util/file_descriptor.h"
 #include "test/util/fs_util.h"
 #include "test/util/temp_path.h"
@@ -1512,6 +1518,55 @@ TEST(Inotify, ControlEvents) {
   ASSERT_THAT(events2, Are({}));
 }
 
+// Regression test to ensure epoll and directory access doesn't deadlock.
+TEST(Inotify, EpollNoDeadlock) {
+  const DisableSave ds;  // Too many syscalls.
+
+  const FileDescriptor fd =
+      ASSERT_NO_ERRNO_AND_VALUE(InotifyInit1(IN_NONBLOCK));
+
+  // Create lots of directories and watch all of them.
+  const TempPath root = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
+  std::vector<TempPath> children;
+  for (size_t i = 0; i < 1000; ++i) {
+    auto child = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDirIn(root.path()));
+    ASSERT_NO_ERRNO_AND_VALUE(
+        InotifyAddWatch(fd.get(), child.path(), IN_ACCESS));
+    children.emplace_back(std::move(child));
+  }
+
+  // Run epoll_wait constantly in a separate thread.
+  std::atomic<bool> done(false);
+  ScopedThread th([&fd, &done] {
+    for (auto start = absl::Now(); absl::Now() - start < absl::Seconds(5);) {
+      FileDescriptor epoll_fd = ASSERT_NO_ERRNO_AND_VALUE(NewEpollFD());
+      ASSERT_NO_ERRNO(RegisterEpollFD(epoll_fd.get(), fd.get(),
+                                      EPOLLIN | EPOLLOUT | EPOLLET, 0));
+      struct epoll_event result[1];
+      EXPECT_THAT(RetryEINTR(epoll_wait)(epoll_fd.get(), result, 1, -1),
+                  SyscallSucceedsWithValue(1));
+
+      sched_yield();
+    }
+    done = true;
+  });
+
+  // While epoll thread is running, constantly access all directories to
+  // generate inotify events.
+  while (!done) {
+    std::vector<std::string> files =
+        ASSERT_NO_ERRNO_AND_VALUE(ListDir(root.path(), false));
+    ASSERT_EQ(files.size(), 1002);
+    for (const auto& child : files) {
+      if (child == "." || child == "..") {
+        continue;
+      }
+      ASSERT_NO_ERRNO_AND_VALUE(ListDir(JoinPath(root.path(), child), false));
+    }
+    sched_yield();
+  }
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace gvisor