[PATCH 4.19 043/273] eventfd: Export eventfd_ctx_do_read()

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4.19-stable review patch.  If anyone has any objections, please let me know.


From: David Woodhouse <dwmw@xxxxxxxxxxxx>

[ 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>
 fs/eventfd.c            | 5 ++++-
 include/linux/eventfd.h | 6 ++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index ce1d1711fbbaf..a96de1f0377bc 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -174,11 +174,14 @@ 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;
  * 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 /* _LINUX_EVENTFD_H */

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