Subject: [PATCH v14 2/8] net: introduce napi_busy_loop_rcu()

From: Stefan Roesch <[email-masked]>
Date: Mon, 5 Jun 2023 14:20:03 -0700

This introduces the napi_busy_loop_rcu() function. If the caller of
napi_busy_loop() function is also taking the rcu read lock, it is possible
that napi_busy_loop() is releasing the read lock if it invokes schedule.
However the caller is expecting that the rcu read lock is not released
until the function completes. This new function avoids that problem. It
expects that the caller MUST hold the rcu_read_lock while calling this
function.

Signed-off-by: Stefan Roesch <[email-masked]>
---
 include/net/busy_poll.h |  4 ++++
 net/core/dev.c          | 31 +++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index f90f0021f5f2..622623f5740e 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -47,6 +47,10 @@ 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_busy_loop_rcu(unsigned int napi_id,
+			bool (*loop_end)(void *, unsigned long),
+			void *loop_end_arg, bool prefer_busy_poll, u16 budget);
+
 #else /* CONFIG_NET_RX_BUSY_POLL */
 static inline unsigned long net_busy_loop_on(void)
 {
diff --git a/net/core/dev.c b/net/core/dev.c
index f4677aa20f84..fcd4a6a70646 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6213,6 +6213,37 @@ static inline void __napi_busy_poll(struct napi_busy_poll_ctx *ctx,
 				LINUX_MIB_BUSYPOLLRXPACKETS, work);
 	local_bh_enable();
 }
+
+/*
+ * Warning: can exit without calling need_resched().
+ */
+void napi_busy_loop_rcu(unsigned int napi_id,
+		    bool (*loop_end)(void *, unsigned long),
+		    void *loop_end_arg, bool prefer_busy_poll, u16 budget)
+{
+	unsigned long start_time = loop_end ? busy_loop_current_time() : 0;
+	struct napi_busy_poll_ctx ctx = {};
+
+	ctx.napi = napi_by_id(napi_id);
+	if (!ctx.napi)
+		return;
+
+	preempt_disable();
+	for (;;) {
+		__napi_busy_poll(&ctx, prefer_busy_poll, budget);
+
+		if (!loop_end || loop_end(loop_end_arg, start_time))
+			break;
+		if (unlikely(need_resched()))
+			break;
+
+		cpu_relax();
+	}
+	if (ctx.napi_poll)
+		busy_poll_stop(ctx.napi, ctx.have_poll_lock, prefer_busy_poll, budget);
+	preempt_enable();
+}
+
 void napi_busy_loop(unsigned int napi_id,
 		    bool (*loop_end)(void *, unsigned long),
 		    void *loop_end_arg, bool prefer_busy_poll, u16 budget)
-- 
2.39.1