linux-kernel - [PATCH 4.4 79/90] net: better skb->sender_cpu and skb->napi

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Message-Id: <20170612152601.814744187@linuxfoundation.org>
Date:   Mon, 12 Jun 2017 17:26:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paul Menzel <pmenzel@...gen.mpg.de>
Subject: [PATCH 4.4 79/90] net: better skb->sender_cpu and skb->napi_id cohabitation

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

------------------

From: Eric Dumazet <edumazet@...gle.com>

commit 52bd2d62ce6758d811edcbd2256eb9ea7f6a56cb upstream.

skb->sender_cpu and skb->napi_id share a common storage,
and we had various bugs about this.

We had to call skb_sender_cpu_clear() in some places to
not leave a prior skb->napi_id and fool netdev_pick_tx()

As suggested by Alexei, we could split the space so that
these errors can not happen.

0 value being reserved as the common (not initialized) value,
let's reserve [1 .. NR_CPUS] range for valid sender_cpu,
and [NR_CPUS+1 .. ~0U] for valid napi_id.

This will allow proper busy polling support over tunnels.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Suggested-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Cc: Paul Menzel <pmenzel@...gen.mpg.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 include/linux/skbuff.h |    3 ---
 net/core/dev.c         |   33 ++++++++++++++++-----------------
 2 files changed, 16 insertions(+), 20 deletions(-)

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1084,9 +1084,6 @@ static inline void skb_copy_hash(struct
 
 static inline void skb_sender_cpu_clear(struct sk_buff *skb)
 {
-#ifdef CONFIG_XPS
-	skb->sender_cpu = 0;
-#endif
 }
 
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -182,7 +182,7 @@ EXPORT_SYMBOL(dev_base_lock);
 /* protects napi_hash addition/deletion and napi_gen_id */
 static DEFINE_SPINLOCK(napi_hash_lock);
 
-static unsigned int napi_gen_id;
+static unsigned int napi_gen_id = NR_CPUS;
 static DEFINE_HASHTABLE(napi_hash, 8);
 
 static seqcount_t devnet_rename_seq;
@@ -3049,7 +3049,9 @@ struct netdev_queue *netdev_pick_tx(stru
 	int queue_index = 0;
 
 #ifdef CONFIG_XPS
-	if (skb->sender_cpu == 0)
+	u32 sender_cpu = skb->sender_cpu - 1;
+
+	if (sender_cpu >= (u32)NR_CPUS)
 		skb->sender_cpu = raw_smp_processor_id() + 1;
 #endif
 
@@ -4726,25 +4728,22 @@ EXPORT_SYMBOL_GPL(napi_by_id);
 
 void napi_hash_add(struct napi_struct *napi)
 {
-	if (!test_and_set_bit(NAPI_STATE_HASHED, &napi->state)) {
+	if (test_and_set_bit(NAPI_STATE_HASHED, &napi->state))
+		return;
 
-		spin_lock(&napi_hash_lock);
+	spin_lock(&napi_hash_lock);
 
-		/* 0 is not a valid id, we also skip an id that is taken
-		 * we expect both events to be extremely rare
-		 */
-		napi->napi_id = 0;
-		while (!napi->napi_id) {
-			napi->napi_id = ++napi_gen_id;
-			if (napi_by_id(napi->napi_id))
-				napi->napi_id = 0;
-		}
+	/* 0..NR_CPUS+1 range is reserved for sender_cpu use */
+	do {
+		if (unlikely(++napi_gen_id < NR_CPUS + 1))
+			napi_gen_id = NR_CPUS + 1;
+	} while (napi_by_id(napi_gen_id));
+	napi->napi_id = napi_gen_id;
 
-		hlist_add_head_rcu(&napi->napi_hash_node,
-			&napi_hash[napi->napi_id % HASH_SIZE(napi_hash)]);
+	hlist_add_head_rcu(&napi->napi_hash_node,
+			   &napi_hash[napi->napi_id % HASH_SIZE(napi_hash)]);
 
-		spin_unlock(&napi_hash_lock);
-	}
+	spin_unlock(&napi_hash_lock);
 }
 EXPORT_SYMBOL_GPL(napi_hash_add);