netdev - [PATCH v13 2/7] net: introduce napi_busy_loop

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Date: Thu, 18 May 2023 14:17:46 -0700
From: Stefan Roesch <shr@...kernel.io>
To: io-uring@...r.kernel.org,
	kernel-team@...com
Cc: shr@...kernel.io,
	axboe@...nel.dk,
	ammarfaizi2@...weeb.org,
	netdev@...r.kernel.org,
	kuba@...nel.org,
	olivier@...llion01.com
Subject: [PATCH v13 2/7] net: introduce napi_busy_loop_rcu()

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 <shr@...kernel.io>
---
 include/net/busy_poll.h |  4 ++++
 net/core/dev.c          | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 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..51a1c70a70a2 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6213,6 +6213,44 @@ static inline void __napi_busy_poll(struct napi_busy_poll_ctx *ctx,
 				LINUX_MIB_BUSYPOLLRXPACKETS, work);
 	local_bh_enable();
 }
+
+/*
+ * napi_busy_loop_rcu() - busy poll (caller holds rcu read lock)
+ * @napi_id         : napi id
+ * @loop_end        : function to check for loop end
+ * @loop_end_arg    : argument for loop end function
+ * @prefer_busy_poll: prefer busy poll
+ * @budget          : budget for busy poll
+ *
+ * Warning: caller must hold rcu read lock.
+ */
+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