eventfd: Export eventfd_ctx_do_read()

Where events are consumed in the kernel, for example by KVM's
irqfd_wakeup() and VFIO's virqfd_wakeup(), they currently lack a
mechanism to drain the eventfd's counter.

Since the wait queue is already locked while the wakeup functions are
invoked, all they really need to do is call eventfd_ctx_do_read().

Add a check for the lock, and export it for them.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20201027135523.646811-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

fs/eventfd.c

@@ -182,11 +182,14 @@ static __poll_t eventfd_poll(struct file *file, poll_table *wait)
 	return events;
 }
 
-static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
+void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
 {
+	lockdep_assert_held(&ctx->wqh.lock);
+
 	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
 	ctx->count -= *cnt;
 }
+EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
 
 /**
  * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.

include/linux/eventfd.h

@@ -41,6 +41,7 @@ struct eventfd_ctx *eventfd_ctx_fileget(struct file *file);
 __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);
 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
 				  __u64 *cnt);
+void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt);
 
 DECLARE_PER_CPU(int, eventfd_wake_count);
 
@@ -82,6 +83,11 @@ static inline bool eventfd_signal_count(void)
 	return false;
 }
 
+static inline void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
+{
+
+}
+
 #endif
 
 #endif /* _LINUX_EVENTFD_H */