netdev - [PATCH net-next] page_pool: fix logic in __page_pool_get

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Message-ID: <20190813174509.494723-1-jonathan.lemon@gmail.com>
Date:   Tue, 13 Aug 2019 10:45:09 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     <netdev@...r.kernel.org>, <davem@...emloft.net>
CC:     <brouer@...hat.com>, <ilias.apalodimas@...aro.org>,
        <saeedm@...lanox.com>, <ttoukan.linux@...il.com>,
        <kernel-team@...com>
Subject: [PATCH net-next] page_pool: fix logic in __page_pool_get_cached

__page_pool_get_cached() will return NULL when the ring is
empty, even if there are pages present in the lookaside cache.

It is also possible to refill the cache, and then return a
NULL page.

Restructure the logic so eliminate both cases.

Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
---
 net/core/page_pool.c | 39 ++++++++++++++++-----------------------
 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 68510eb869ea..de09a74a39a4 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -82,12 +82,9 @@ EXPORT_SYMBOL(page_pool_create);
 static struct page *__page_pool_get_cached(struct page_pool *pool)
 {
 	struct ptr_ring *r = &pool->ring;
+	bool refill = false;
 	struct page *page;
 
-	/* Quicker fallback, avoid locks when ring is empty */
-	if (__ptr_ring_empty(r))
-		return NULL;
-
 	/* Test for safe-context, caller should provide this guarantee */
 	if (likely(in_serving_softirq())) {
 		if (likely(pool->alloc.count)) {
@@ -95,27 +92,23 @@ static struct page *__page_pool_get_cached(struct page_pool *pool)
 			page = pool->alloc.cache[--pool->alloc.count];
 			return page;
 		}
-		/* Slower-path: Alloc array empty, time to refill
-		 *
-		 * Open-coded bulk ptr_ring consumer.
-		 *
-		 * Discussion: the ring consumer lock is not really
-		 * needed due to the softirq/NAPI protection, but
-		 * later need the ability to reclaim pages on the
-		 * ring. Thus, keeping the locks.
-		 */
-		spin_lock(&r->consumer_lock);
-		while ((page = __ptr_ring_consume(r))) {
-			if (pool->alloc.count == PP_ALLOC_CACHE_REFILL)
-				break;
-			pool->alloc.cache[pool->alloc.count++] = page;
-		}
-		spin_unlock(&r->consumer_lock);
-		return page;
+		refill = true;
 	}
 
-	/* Slow-path: Get page from locked ring queue */
-	page = ptr_ring_consume(&pool->ring);
+	/* Quicker fallback, avoid locks when ring is empty */
+	if (__ptr_ring_empty(r))
+		return NULL;
+
+	/* Slow-path: Get page from locked ring queue,
+	 * refill alloc array if requested.
+	 */
+	spin_lock(&r->consumer_lock);
+	page = __ptr_ring_consume(r);
+	if (refill)
+		pool->alloc.count = __ptr_ring_consume_batched(r,
+							pool->alloc.cache,
+							PP_ALLOC_CACHE_REFILL);
+	spin_unlock(&r->consumer_lock);
 	return page;
 }
 
-- 
2.17.1