Re: [RFC v4 01/11] eventfd: Increase the recursion depth of eventfd_signal()

[Jason Wang]

On 2021/2/23 7:50 下午, Xie Yongji wrote:

Increase the recursion depth of eventfd_signal() to 1. This
is the maximum recursion depth we have found so far.

Signed-off-by: Xie Yongji <xieyon...@bytedance.com>

Acked-by: Jason Wang <jasow...@redhat.com>

It might be useful to explain how/when we can reach for this condition.

Thanks

---
  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 */