[PATCH v9 net-next 04/15] net: Use nested-BH locking for napi_alloc_cache.

Sebastian Andrzej Siewior
2024-06-20T13:27:33Z

napi_alloc_cache 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.

Add a local_lock_t to the data structure and use local_lock_nested_bh()
for locking. This change adds only lockdep coverage and does not alter
the functional behaviour for !PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
net/core/skbuff.c | 29 ++++++++++++++++++++++++-----
1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1b52f69ad05e2..eb9a7e65b5c81 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -277,6 +277,7 @@ static void *page_frag_alloc_1k(struct page_frag_1k *nc, gfp_t gfp_mask)
#endif

struct napi_alloc_cache {
+ local_lock_t bh_lock;
struct page_frag_cache page;
struct page_frag_1k page_small;
unsigned int skb_count;
@@ -284,7 +285,9 @@ 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 DEFINE_PER_CPU(struct napi_alloc_cache, napi_alloc_cache) = {
+ .bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};

/* Double check that napi_get_frags() allocates skbs with
* skb->head being backed by slab, not a page fragment.
@@ -306,11 +309,16 @@ void napi_get_frags_check(struct napi_struct *napi)
void *__napi_alloc_frag_align(unsigned int fragsz, unsigned int align_mask)
{
struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+ void *data;

fragsz = SKB_DATA_ALIGN(fragsz);

- return __page_frag_alloc_align(&nc->page, fragsz, GFP_ATOMIC,
+ local_lock_nested_bh(&napi_alloc_cache.bh_lock);
+ data = __page_frag_alloc_align(&nc->page, fragsz, GFP_ATOMIC,
align_mask);
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
+ return data;
+
}
EXPORT_SYMBOL(__napi_alloc_frag_align);

@@ -338,16 +346,20 @@ static struct sk_buff *napi_skb_cache_get(void)
struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
struct sk_buff *skb;

+ local_lock_nested_bh(&napi_alloc_cache.bh_lock);
if (unlikely(!nc->skb_count)) {
nc->skb_count = kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
GFP_ATOMIC,
NAPI_SKB_CACHE_BULK,
nc->skb_cache);
- if (unlikely(!nc->skb_count))
+ if (unlikely(!nc->skb_count)) {
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
return NULL;
+ }
}

skb = nc->skb_cache[--nc->skb_count];
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
kasan_mempool_unpoison_object(skb, kmem_cache_size(net_hotdata.skbuff_cache));

return skb;
@@ -740,9 +752,13 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev, unsigned int len,
pfmemalloc = nc->pfmemalloc;
} else {
local_bh_disable();
+ local_lock_nested_bh(&napi_alloc_cache.bh_lock);
+
nc = this_cpu_ptr(&napi_alloc_cache.page);
data = page_frag_alloc(nc, len, gfp_mask);
pfmemalloc = nc->pfmemalloc;
+
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
local_bh_enable();
}

@@ -806,11 +822,11 @@ struct sk_buff *napi_alloc_skb(struct napi_struct *napi, unsigned int len)
goto skb_success;
}

- nc = this_cpu_ptr(&napi_alloc_cache);
-
if (sk_memalloc_socks())
gfp_mask |= __GFP_MEMALLOC;

+ local_lock_nested_bh(&napi_alloc_cache.bh_lock);
+ nc = this_cpu_ptr(&napi_alloc_cache);
if (NAPI_HAS_SMALL_PAGE_FRAG && len <= SKB_WITH_OVERHEAD(1024)) {
/* we are artificially inflating the allocation size, but
* that is not as bad as it may look like, as:
@@ -832,6 +848,7 @@ struct sk_buff *napi_alloc_skb(struct napi_struct *napi, unsigned int len)
data = page_frag_alloc(&nc->page, len, gfp_mask);
pfmemalloc = nc->page.pfmemalloc;
}
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);

if (unlikely(!data))
return NULL;
@@ -1431,6 +1448,7 @@ static void napi_skb_cache_put(struct sk_buff *skb)
if (!kasan_mempool_poison_object(skb))
return;

+ local_lock_nested_bh(&napi_alloc_cache.bh_lock);
nc->skb_cache[nc->skb_count++] = skb;

if (unlikely(nc->skb_count == NAPI_SKB_CACHE_SIZE)) {
@@ -1442,6 +1460,7 @@ static void napi_skb_cache_put(struct sk_buff *skb)
nc->skb_cache + NAPI_SKB_CACHE_HALF);
nc->skb_count = NAPI_SKB_CACHE_HALF;
}
+ local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
}

void __napi_kfree_skb(struct sk_buff *skb, enum skb_drop_reason reason)
--
2.45.2