netdev - [PATCH v2 net 4/5] mlx4: fix race accessing page->

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Date:	Fri, 10 Oct 2014 04:48:17 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v2 net 4/5] mlx4: fix race accessing page->_count

This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 drivers/net/ethernet/mellanox/mlx4/en_rx.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index a33048ee9621..01660c595f5c 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -76,10 +76,10 @@ static int mlx4_alloc_pages(struct mlx4_en_priv *priv,
 	page_alloc->dma = dma;
 	page_alloc->page_offset = frag_info->frag_align;
 	/* Not doing get_page() for each frag is a big win
-	 * on asymetric workloads.
+	 * on asymetric workloads. Note we can not use atomic_set().
 	 */
-	atomic_set(&page->_count,
-		   page_alloc->page_size / frag_info->frag_stride);
+	atomic_add(page_alloc->page_size / frag_info->frag_stride - 1,
+		   &page->_count);
 	return 0;
 }
 
-- 
2.1.0.rc2.206.gedb03e5

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