Re: [PATCH net-next v3 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@lunn.ch>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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 2d11d013cabed..2018e2432cb56 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -3502,7 +3502,12 @@ 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 1be7cb73a6024..b56becd070bc7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -462,7 +462,9 @@ 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
 /*

@@ -5238,7 +5240,10 @@ 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,

@@ -12629,7 +12634,7 @@ 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;
 }

@@ -12759,13 +12764,13 @@ 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 f86eedad586a7..b2a5c934fe7b7 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c

@@ -737,10 +737,10 @@ 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;

@@ -748,13 +748,18 @@ 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;
 	}
 

@@ -773,9 +778,11 @@ 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);