Re: [PATCH RFC bpf-next 4/4] i40e: remove rcu_read_lock() around XDP program invocation
From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Date: 2021-04-23 14:11:53
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bpf
On Fri, Apr 23, 2021 at 01:05:20PM +0200, Toke Høiland-Jørgensen wrote:
From: Toke Høiland-Jørgensen <redacted> The i40e driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP program invocations. However, the actual lifetime of the objects referred by the XDP program invocation is longer, all the way through to the call to xdp_do_flush(), making the scope of the rcu_read_lock() too small. This turns out to be harmless because it all happens in a single NAPI poll cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock() misleading.
Okay, but what about the lifetime of the xdp_prog itself? Can xdp_prog change within a single NAPI poll? After reading previous discussions I would say it can't, right? There are drivers that have a big RCU critical section in NAPI poll, but it seems that some read a xdp_prog a single time whereas others read it per processed frame. If we are sure that xdp_prog can't change on-the-fly then first low hanging fruit, at least for the Intel drivers, is to skip a test against NULL and read it only once at the beginning of NAPI poll. There might be also other micro-optimizations specific to each drivers that could be done based on that (that of course read the xdp_prog per each frame). Or am I nuts?
quoted hunk ↗ jump to hunk
Rather than extend the scope of the rcu_read_lock(), just get rid of it entirely. With the addition of RCU annotations to the XDP_REDIRECT map types that take bh execution into account, lockdep even understands this to be safe, so there's really no reason to keep it around. Signed-off-by: Toke Høiland-Jørgensen <redacted> --- drivers/net/ethernet/intel/i40e/i40e_txrx.c | 2 -- drivers/net/ethernet/intel/i40e/i40e_xsk.c | 6 +----- 2 files changed, 1 insertion(+), 7 deletions(-)diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c index fc20afc23bfa..3f4c947a5185 100644 --- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c +++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c@@ -2303,7 +2303,6 @@ static struct sk_buff *i40e_run_xdp(struct i40e_ring *rx_ring, struct bpf_prog *xdp_prog; u32 act; - rcu_read_lock(); xdp_prog = READ_ONCE(rx_ring->xdp_prog); if (!xdp_prog)@@ -2334,7 +2333,6 @@ static struct sk_buff *i40e_run_xdp(struct i40e_ring *rx_ring, break; } xdp_out: - rcu_read_unlock(); return ERR_PTR(-result); }diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c index d89c22347d9d..93b349f11d3b 100644 --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c@@ -153,7 +153,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp) struct bpf_prog *xdp_prog; u32 act; - rcu_read_lock(); /* NB! xdp_prog will always be !NULL, due to the fact that * this path is enabled by setting an XDP program. */@@ -162,9 +161,7 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp) if (likely(act == XDP_REDIRECT)) { err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog); - result = !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED; - rcu_read_unlock(); - return result; + return !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED; } switch (act) {@@ -184,7 +181,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp) result = I40E_XDP_CONSUMED; break; } - rcu_read_unlock(); return result; }