[PATCH v6 02/10] eventfd: Increase the recursion depth of eventfd_signal()

[Xie Yongji <xieyongji@bytedance.com>]

Increase the recursion depth of eventfd_signal() to 1. This
is the maximum recursion depth we have found so far, which
can be triggered with the following call chain:

    kvm_io_bus_write                        [kvm]
      --> ioeventfd_write                   [kvm]
        --> eventfd_signal                  [eventfd]
          --> vhost_poll_wakeup             [vhost]
            --> vduse_vdpa_kick_vq          [vduse]
              --> eventfd_signal            [eventfd]

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
---
 fs/eventfd.c            | 2 +-
 include/linux/eventfd.h | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index e265b6dd4f34..cc7cd1dbedd3 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -71,7 +71,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
 	 * it returns true, the eventfd_signal() call should be deferred to a
 	 * safe context.
 	 */
-	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
+	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH))
 		return 0;
 
 	spin_lock_irqsave(&ctx->wqh.lock, flags);
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index fa0a524baed0..886d99cd38ef 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -29,6 +29,9 @@
 #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
 #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
 
+/* Maximum recursion depth */
+#define EFD_WAKE_DEPTH 1
+
 struct eventfd_ctx;
 struct file;
 
@@ -47,7 +50,7 @@ DECLARE_PER_CPU(int, eventfd_wake_count);
 
 static inline bool eventfd_signal_count(void)
 {
-	return this_cpu_read(eventfd_wake_count);
+	return this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH;
 }
 
 #else /* CONFIG_EVENTFD */
-- 
2.11.0