netdev - [PATCH net-next 05/18] xdp: Use nested-BH locking for system_page

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Message-ID: <20250309144653.825351-6-bigeasy@linutronix.de>
Date: Sun,  9 Mar 2025 15:46:40 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: netdev@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>
Subject: [PATCH net-next 05/18] xdp: Use nested-BH locking for system_page_pool.

system_page_pool is a per-CPU variable and relies on disabled BH for its
locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Make a struct with a page_pool member (original system_page_pool) and a
local_lock_t and use local_lock_nested_bh() for locking. This change
adds only lockdep coverage and does not alter the functional behaviour
for !PREEMPT_RT.

Cc: Andrew Lunn <andrew+netdev@...n.ch>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>
Cc: John Fastabend <john.fastabend@...il.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/netdevice.h |  7 ++++++-
 net/core/dev.c            | 15 ++++++++++-----
 net/core/xdp.c            | 11 +++++++++--
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index ab550a89b9bfa..6b740a4d7ee6d 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3465,7 +3465,12 @@ struct softnet_data {
 };
 
 DECLARE_PER_CPU_ALIGNED(struct softnet_data, softnet_data);
-DECLARE_PER_CPU(struct page_pool *, system_page_pool);
+
+struct page_pool_bh {
+	struct page_pool *pool;
+	local_lock_t bh_lock;
+};
+DECLARE_PER_CPU(struct page_pool_bh, system_page_pool);
 
 #ifndef CONFIG_PREEMPT_RT
 static inline int dev_recursion_level(void)
diff --git a/net/core/dev.c b/net/core/dev.c
index 30da277c5a6f8..a9955b1c33001 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -460,7 +460,9 @@ EXPORT_PER_CPU_SYMBOL(softnet_data);
  * PP consumers must pay attention to run APIs in the appropriate context
  * (e.g. NAPI context).
  */
-DEFINE_PER_CPU(struct page_pool *, system_page_pool);
+DEFINE_PER_CPU(struct page_pool_bh, system_page_pool) = {
+	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};
 
 #ifdef CONFIG_LOCKDEP
 /*
@@ -5253,7 +5255,10 @@ netif_skb_check_for_xdp(struct sk_buff **pskb, const struct bpf_prog *prog)
 	struct sk_buff *skb = *pskb;
 	int err, hroom, troom;
 
-	if (!skb_cow_data_for_xdp(this_cpu_read(system_page_pool), pskb, prog))
+	local_lock_nested_bh(&system_page_pool.bh_lock);
+	err = skb_cow_data_for_xdp(this_cpu_read(system_page_pool.pool), pskb, prog);
+	local_unlock_nested_bh(&system_page_pool.bh_lock);
+	if (!err)
 		return 0;
 
 	/* In case we have to go down the path and also linearize,
@@ -12522,7 +12527,7 @@ static int net_page_pool_create(int cpuid)
 		return err;
 	}
 
-	per_cpu(system_page_pool, cpuid) = pp_ptr;
+	per_cpu(system_page_pool.pool, cpuid) = pp_ptr;
 #endif
 	return 0;
 }
@@ -12652,13 +12657,13 @@ static int __init net_dev_init(void)
 		for_each_possible_cpu(i) {
 			struct page_pool *pp_ptr;
 
-			pp_ptr = per_cpu(system_page_pool, i);
+			pp_ptr = per_cpu(system_page_pool.pool, i);
 			if (!pp_ptr)
 				continue;
 
 			xdp_unreg_page_pool(pp_ptr);
 			page_pool_destroy(pp_ptr);
-			per_cpu(system_page_pool, i) = NULL;
+			per_cpu(system_page_pool.pool, i) = NULL;
 		}
 	}
 
diff --git a/net/core/xdp.c b/net/core/xdp.c
index 2c6ab6fb452f7..4b09c96e384dc 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -747,10 +747,10 @@ static noinline bool xdp_copy_frags_from_zc(struct sk_buff *skb,
  */
 struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 {
-	struct page_pool *pp = this_cpu_read(system_page_pool);
 	const struct xdp_rxq_info *rxq = xdp->rxq;
 	u32 len = xdp->data_end - xdp->data_meta;
 	u32 truesize = xdp->frame_sz;
+	struct page_pool *pp;
 	struct sk_buff *skb;
 	int metalen;
 	void *data;
@@ -758,13 +758,18 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 	if (!IS_ENABLED(CONFIG_PAGE_POOL))
 		return NULL;
 
+	local_lock_nested_bh(&system_page_pool.bh_lock);
+	pp = this_cpu_read(system_page_pool.pool);
 	data = page_pool_dev_alloc_va(pp, &truesize);
-	if (unlikely(!data))
+	if (unlikely(!data)) {
+		local_unlock_nested_bh(&system_page_pool.bh_lock);
 		return NULL;
+	}
 
 	skb = napi_build_skb(data, truesize);
 	if (unlikely(!skb)) {
 		page_pool_free_va(pp, data, true);
+		local_unlock_nested_bh(&system_page_pool.bh_lock);
 		return NULL;
 	}
 
@@ -783,9 +788,11 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 
 	if (unlikely(xdp_buff_has_frags(xdp)) &&
 	    unlikely(!xdp_copy_frags_from_zc(skb, xdp, pp))) {
+		local_unlock_nested_bh(&system_page_pool.bh_lock);
 		napi_consume_skb(skb, true);
 		return NULL;
 	}
+	local_unlock_nested_bh(&system_page_pool.bh_lock);
 
 	xsk_buff_free(xdp);
 
-- 
2.47.2