Patch "eventfd: Export eventfd_ctx_do_read()" has been added to the 5.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    eventfd: Export eventfd_ctx_do_read()

to the 5.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     eventfd-export-eventfd_ctx_do_read.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 85ababaf725eac4731210e70850f5c4630293644
Author: David Woodhouse <dwmw@xxxxxxxxxxxx>
Date:   Tue Oct 27 13:55:21 2020 +0000

    eventfd: Export eventfd_ctx_do_read()
    
    [ Upstream commit 28f1326710555bbe666f64452d08f2d7dd657cae ]
    
    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@xxxxxxxxxxxx>
    Message-Id: <20201027135523.646811-2-dwmw2@xxxxxxxxxxxxx>
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    Stable-dep-of: 758b49204781 ("eventfd: prevent underflow for eventfd semaphores")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 78e41c7c3d05b..26b3d821e9168 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -181,11 +181,14 @@ 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.
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index 3482f9365a4db..de0ad39d4281f 100644
--- a/include/linux/eventfd.h
+++ b/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 */