[RFC PATCH v3 16/20] net: execute custom callback from napi

[David Wei <dw@davidwei.uk>]

From: Pavel Begunkov <asml.silence@gmail.com>

Sometimes we want to access a napi protected resource from task
context like in the case of io_uring zc falling back to copy and
accessing the buffer ring. Add a helper function that allows to execute
a custom function from napi context by first stopping it similarly to
napi_busy_loop().

Experimental, needs much polishing and sharing bits with
napi_busy_loop().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David Wei <dw@davidwei.uk>
---
 include/net/busy_poll.h |  7 +++++++
 net/core/dev.c          | 46 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index 4dabeb6c76d3..64238467e00a 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -47,6 +47,8 @@ bool sk_busy_loop_end(void *p, unsigned long start_time);
 void napi_busy_loop(unsigned int napi_id,
 		    bool (*loop_end)(void *, unsigned long),
 		    void *loop_end_arg, bool prefer_busy_poll, u16 budget);
+void napi_execute(struct napi_struct *napi,
+		  void (*cb)(void *), void *cb_arg);
 
 #else /* CONFIG_NET_RX_BUSY_POLL */
 static inline unsigned long net_busy_loop_on(void)
@@ -59,6 +61,11 @@ static inline bool sk_can_busy_loop(struct sock *sk)
 	return false;
 }
 
+static inline void napi_execute(struct napi_struct *napi,
+				void (*cb)(void *), void *cb_arg)
+{
+}
+
 #endif /* CONFIG_NET_RX_BUSY_POLL */
 
 static inline unsigned long busy_loop_current_time(void)
diff --git a/net/core/dev.c b/net/core/dev.c
index e55750c47245..2dd4f3846535 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6537,6 +6537,52 @@ void napi_busy_loop(unsigned int napi_id,
 }
 EXPORT_SYMBOL(napi_busy_loop);
 
+void napi_execute(struct napi_struct *napi,
+		  void (*cb)(void *), void *cb_arg)
+{
+	bool done = false;
+	unsigned long val;
+	void *have_poll_lock = NULL;
+
+	rcu_read_lock();
+
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
+	for (;;) {
+		local_bh_disable();
+		val = READ_ONCE(napi->state);
+
+		/* If multiple threads are competing for this napi,
+		* we avoid dirtying napi->state as much as we can.
+		*/
+		if (val & (NAPIF_STATE_DISABLE | NAPIF_STATE_SCHED |
+			  NAPIF_STATE_IN_BUSY_POLL))
+			goto restart;
+
+		if (cmpxchg(&napi->state, val,
+			   val | NAPIF_STATE_IN_BUSY_POLL |
+				 NAPIF_STATE_SCHED) != val)
+			goto restart;
+
+		have_poll_lock = netpoll_poll_lock(napi);
+		cb(cb_arg);
+		done = true;
+		gro_normal_list(napi);
+		local_bh_enable();
+		break;
+restart:
+		local_bh_enable();
+		if (unlikely(need_resched()))
+			break;
+		cpu_relax();
+	}
+	if (done)
+		busy_poll_stop(napi, have_poll_lock, false, 1);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+	rcu_read_unlock();
+}
+
 #endif /* CONFIG_NET_RX_BUSY_POLL */
 
 static void napi_hash_add(struct napi_struct *napi)
-- 
2.39.3