[net-next v1 PATCH 3/4] page_pool: block alloc cache during shutdown

[Jesper Dangaard Brouer <brouer@redhat.com>]

It is documented that driver API users, have to disconnect
the page_pool, before starting shutdown phase, but is it only
documentation, there is not code catching such violations.

Given (in page_pool_empty_alloc_cache_once) alloc cache is only
flushed once, there is now an opportunity to catch this case.

This patch blocks the RX/alloc-side cache, via pretending it is
full and poison last element.  This code change will enforce that
drivers cannot use alloc cache during shutdown phase.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/page_pool.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index e28db2ef8e12..b31f3bb7818d 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -364,6 +364,10 @@ static void page_pool_empty_alloc_cache_once(struct page_pool *pool)
 		page = pool->alloc.cache[--pool->alloc.count];
 		__page_pool_return_page(pool, page);
 	}
+
+	/* Block alloc cache, pretend it's full and poison last element */
+	pool->alloc.cache[PP_ALLOC_CACHE_SIZE - 1] = NULL;
+	pool->alloc.count = PP_ALLOC_CACHE_SIZE;
 }
 
 static void page_pool_scrub(struct page_pool *pool)