[RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases

[Xie Yongji <xieyongji@xxxxxxxxxxxxx>]

Now we have a global percpu counter to limit the recursion depth
of eventfd_signal(). This can avoid deadlock or stack overflow.
But in stack overflow case, it should be OK to increase the
recursion depth if needed. So we add a percpu counter in eventfd_ctx
to limit the recursion depth for deadlock case. Then it could be
fine to increase the global percpu counter later.

Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
---
 fs/aio.c                |  3 ++-
 fs/eventfd.c            | 20 +++++++++++++++++++-
 include/linux/eventfd.h |  5 +----
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 1f32da13d39e..5d82903161f5 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1698,7 +1698,8 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 		list_del(&iocb->ki_list);
 		iocb->ki_res.res = mangle_poll(mask);
 		req->done = true;
-		if (iocb->ki_eventfd && eventfd_signal_count()) {
+		if (iocb->ki_eventfd &&
+			eventfd_signal_count(iocb->ki_eventfd)) {
 			iocb = NULL;
 			INIT_WORK(&req->work, aio_poll_put_work);
 			schedule_work(&req->work);
diff --git a/fs/eventfd.c b/fs/eventfd.c
index e265b6dd4f34..2df24f9bada3 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -25,6 +25,8 @@
 #include <linux/idr.h>
 #include <linux/uio.h>
 
+#define EVENTFD_WAKE_DEPTH 0
+
 DEFINE_PER_CPU(int, eventfd_wake_count);
 
 static DEFINE_IDA(eventfd_ida);
@@ -42,9 +44,17 @@ struct eventfd_ctx {
 	 */
 	__u64 count;
 	unsigned int flags;
+	int __percpu *wake_count;
 	int id;
 };
 
+bool eventfd_signal_count(struct eventfd_ctx *ctx)
+{
+	return (this_cpu_read(*ctx->wake_count) ||
+		this_cpu_read(eventfd_wake_count) > EVENTFD_WAKE_DEPTH);
+}
+EXPORT_SYMBOL_GPL(eventfd_signal_count);
+
 /**
  * eventfd_signal - Adds @n to the eventfd counter.
  * @ctx: [in] Pointer to the eventfd context.
@@ -71,17 +81,19 @@ __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(eventfd_signal_count(ctx)))
 		return 0;
 
 	spin_lock_irqsave(&ctx->wqh.lock, flags);
 	this_cpu_inc(eventfd_wake_count);
+	this_cpu_inc(*ctx->wake_count);
 	if (ULLONG_MAX - ctx->count < n)
 		n = ULLONG_MAX - ctx->count;
 	ctx->count += n;
 	if (waitqueue_active(&ctx->wqh))
 		wake_up_locked_poll(&ctx->wqh, EPOLLIN);
 	this_cpu_dec(eventfd_wake_count);
+	this_cpu_dec(*ctx->wake_count);
 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 
 	return n;
@@ -92,6 +104,7 @@ static void eventfd_free_ctx(struct eventfd_ctx *ctx)
 {
 	if (ctx->id >= 0)
 		ida_simple_remove(&eventfd_ida, ctx->id);
+	free_percpu(ctx->wake_count);
 	kfree(ctx);
 }
 
@@ -423,6 +436,11 @@ static int do_eventfd(unsigned int count, int flags)
 
 	kref_init(&ctx->kref);
 	init_waitqueue_head(&ctx->wqh);
+	ctx->wake_count = alloc_percpu(int);
+	if (!ctx->wake_count) {
+		kfree(ctx);
+		return -ENOMEM;
+	}
 	ctx->count = count;
 	ctx->flags = flags;
 	ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index fa0a524baed0..1a11ebbd74a9 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -45,10 +45,7 @@ void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt);
 
 DECLARE_PER_CPU(int, eventfd_wake_count);
 
-static inline bool eventfd_signal_count(void)
-{
-	return this_cpu_read(eventfd_wake_count);
-}
+bool eventfd_signal_count(struct eventfd_ctx *ctx);
 
 #else /* CONFIG_EVENTFD */
 
-- 
2.11.0