[PATCH RT 1/2] net/core: protect users of napi_alloc_cache against reentrance

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On -RT the code running in BH can not be moved to another CPU so CPU
local variable remain local. However the code can be preempted
and another task may enter BH accessing the same CPU using the same
napi_alloc_cache variable.
This patch ensures that each user of napi_alloc_cache uses a local lock.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 net/core/skbuff.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d507656e98ce..7c422e441f1c 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -351,6 +351,7 @@ EXPORT_SYMBOL(build_skb);
 static DEFINE_PER_CPU(struct page_frag_cache, netdev_alloc_cache);
 static DEFINE_PER_CPU(struct page_frag_cache, napi_alloc_cache);
 static DEFINE_LOCAL_IRQ_LOCK(netdev_alloc_lock);
+static DEFINE_LOCAL_IRQ_LOCK(napi_alloc_cache_lock);
 
 static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
 {
@@ -380,9 +381,13 @@ EXPORT_SYMBOL(netdev_alloc_frag);
 
 static void *__napi_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
 {
-	struct page_frag_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+	struct page_frag_cache *nc;
+	void *data;
 
-	return __alloc_page_frag(nc, fragsz, gfp_mask);
+	nc = &get_locked_var(napi_alloc_cache_lock, napi_alloc_cache);
+	data = __alloc_page_frag(nc, fragsz, gfp_mask);
+	put_locked_var(napi_alloc_cache_lock, napi_alloc_cache);
+	return data;
 }
 
 void *napi_alloc_frag(unsigned int fragsz)
@@ -476,9 +481,10 @@ EXPORT_SYMBOL(__netdev_alloc_skb);
 struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
 				 gfp_t gfp_mask)
 {
-	struct page_frag_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+	struct page_frag_cache *nc;
 	struct sk_buff *skb;
 	void *data;
+	bool pfmemalloc;
 
 	len += NET_SKB_PAD + NET_IP_ALIGN;
 
@@ -496,7 +502,11 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
 	if (sk_memalloc_socks())
 		gfp_mask |= __GFP_MEMALLOC;
 
+	nc = &get_locked_var(napi_alloc_cache_lock, napi_alloc_cache);
 	data = __alloc_page_frag(nc, len, gfp_mask);
+	pfmemalloc = nc->pfmemalloc;
+	put_locked_var(napi_alloc_cache_lock, napi_alloc_cache);
+
 	if (unlikely(!data))
 		return NULL;
 
@@ -507,7 +517,7 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
 	}
 
 	/* use OR instead of assignment to avoid clearing of bits in mask */
-	if (nc->pfmemalloc)
+	if (pfmemalloc)
 		skb->pfmemalloc = 1;
 	skb->head_frag = 1;
 
-- 
2.7.0.rc3