net: Don't disable interrupts in napi_alloc_frag()

netdev_alloc_frag() can be used from any context and is used by NAPI
and non-NAPI drivers. Non-NAPI drivers use it in interrupt context
and NAPI drivers use it during initial allocation (->ndo_open() or
->ndo_change_mtu()). Some NAPI drivers share the same function for the
initial allocation and the allocation in their NAPI callback.

The interrupts are disabled in order to ensure locked access from every
context to `netdev_alloc_cache'.

Let netdev_alloc_frag() check if interrupts are disabled. If they are,
use `netdev_alloc_cache' otherwise disable BH and invoke
__napi_alloc_frag() for the allocation. The IRQ check is cheaper
compared to disabling & enabling interrupts and memory allocation with
disabled interrupts does not work on -RT.
Signed-off-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -366,19 +366,21 @@ struct napi_alloc_cache {
 static DEFINE_PER_CPU(struct page_frag_cache, netdev_alloc_cache);
 static DEFINE_PER_CPU(struct napi_alloc_cache, napi_alloc_cache);
 
-static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
+static void *__napi_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
 {
-	struct page_frag_cache *nc;
-	unsigned long flags;
-	void *data;
+	struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
 
-	local_irq_save(flags);
-	nc = this_cpu_ptr(&netdev_alloc_cache);
-	data = page_frag_alloc(nc, fragsz, gfp_mask);
-	local_irq_restore(flags);
-	return data;
+	return page_frag_alloc(&nc->page, fragsz, gfp_mask);
 }
 
+void *napi_alloc_frag(unsigned int fragsz)
+{
+	fragsz = SKB_DATA_ALIGN(fragsz);
+
+	return __napi_alloc_frag(fragsz, GFP_ATOMIC);
+}
+EXPORT_SYMBOL(napi_alloc_frag);
+
 /**
  * netdev_alloc_frag - allocate a page fragment
  * @fragsz: fragment size
@@ -388,26 +390,21 @@ static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
  */
 void *netdev_alloc_frag(unsigned int fragsz)
 {
-	fragsz = SKB_DATA_ALIGN(fragsz);
-
-	return __netdev_alloc_frag(fragsz, GFP_ATOMIC);
-}
-EXPORT_SYMBOL(netdev_alloc_frag);
-
-static void *__napi_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
-{
-	struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
-
-	return page_frag_alloc(&nc->page, fragsz, gfp_mask);
-}
+	struct page_frag_cache *nc;
+	void *data;
 
-void *napi_alloc_frag(unsigned int fragsz)
-{
 	fragsz = SKB_DATA_ALIGN(fragsz);
-
-	return __napi_alloc_frag(fragsz, GFP_ATOMIC);
+	if (in_irq() || irqs_disabled()) {
+		nc = this_cpu_ptr(&netdev_alloc_cache);
+		data = page_frag_alloc(nc, fragsz, GFP_ATOMIC);
+	} else {
+		local_bh_disable();
+		data = __napi_alloc_frag(fragsz, GFP_ATOMIC);
+		local_bh_enable();
+	}
+	return data;
 }
-EXPORT_SYMBOL(napi_alloc_frag);
+EXPORT_SYMBOL(netdev_alloc_frag);
 
 /**
  *	__netdev_alloc_skb - allocate an skbuff for rx on a specific device