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Date: Thu, 9 Oct 2014 22:55:24 -0700
From: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Alexander Duyck <alexander.h.duyck@...hat.com>,
Andres Lagar-Cavilla <andreslc@...gle.com>,
Greg Thelen <gthelen@...gle.com>,
Hugh Dickins <hughd@...gle.com>,
David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH net 2/5] igb: fix race accessing page->_count
On Thu, Oct 9, 2014 at 10:24 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> This is illegal to use atomic_set(&page->_count, 2) 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>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Since this is apart of a series, if the changes to skbuff are ok, then
the changes to the Intel drivers are ok.
> ---
> drivers/net/ethernet/intel/igb/igb_main.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index ae59c0b108c5..a21b14495ebd 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -6545,11 +6545,10 @@ static bool igb_can_reuse_rx_page(struct igb_rx_buffer *rx_buffer,
> /* flip page offset to other buffer */
> rx_buffer->page_offset ^= IGB_RX_BUFSZ;
>
> - /* since we are the only owner of the page and we need to
> - * increment it, just set the value to 2 in order to avoid
> - * an unnecessary locked operation
> + /* Even if we own the page, we are not allowed to use atomic_set()
> + * This would break get_page_unless_zero() users.
> */
> - atomic_set(&page->_count, 2);
> + atomic_inc(&page->_count);
> #else
> /* move offset up to the next cache line */
> rx_buffer->page_offset += truesize;
> --
> 2.1.0.rc2.206.gedb03e5
>
> --
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--
Cheers,
Jeff
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