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authorMagnus Karlsson <magnus.karlsson@intel.com>2018-06-29 09:48:20 +0200
committerAlexei Starovoitov <ast@kernel.org>2018-07-02 18:37:12 -0700
commita9744f7ca200c756e6f8c65b633770a2da711651 (patch)
treee8ab093eacf899194e27a120f8ede438ca3fece3
parentc03079c9d93d593d44524883b6b6127b21978e22 (diff)
downloadlinux-stericsson-a9744f7ca200c756e6f8c65b633770a2da711651.tar.gz
xsk: fix potential race in SKB TX completion code
There is a potential race in the TX completion code for the SKB case. One process enters the sendmsg code of an AF_XDP socket in order to send a frame. The execution eventually trickles down to the driver that is told to send the packet. However, it decides to drop the packet due to some error condition (e.g., rings full) and frees the SKB. This will trigger the SKB destructor and a completion will be sent to the AF_XDP user space through its single-producer/single-consumer queues. At the same time a TX interrupt has fired on another core and it dispatches the TX completion code in the driver. It does its HW specific things and ends up freeing the SKB associated with the transmitted packet. This will trigger the SKB destructor and a completion will be sent to the AF_XDP user space through its single-producer/single-consumer queues. With a pseudo call stack, it would look like this: Core 1: sendmsg() being called in the application netdev_start_xmit() Driver entered through ndo_start_xmit Driver decides to free the SKB for some reason (e.g., rings full) Destructor of SKB called xskq_produce_addr() is called to signal completion to user space Core 2: TX completion irq NAPI loop Driver irq handler for TX completions Frees the SKB Destructor of SKB called xskq_produce_addr() is called to signal completion to user space We now have a violation of the single-producer/single-consumer principle for our queues as there are two threads trying to produce at the same time on the same queue. Fixed by introducing a spin_lock in the destructor. In regards to the performance, I get around 1.74 Mpps for txonly before and after the introduction of the spinlock. There is of course some impact due to the spin lock but it is in the less significant digits that are too noisy for me to measure. But let us say that the version without the spin lock got 1.745 Mpps in the best case and the version with 1.735 Mpps in the worst case, then that would mean a maximum drop in performance of 0.5%. Fixes: 35fcde7f8deb ("xsk: support for Tx") Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
-rw-r--r--include/net/xdp_sock.h4
-rw-r--r--net/xdp/xsk.c4
2 files changed, 8 insertions, 0 deletions
diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index 9fe472f2ac95..7161856bcf9c 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -60,6 +60,10 @@ struct xdp_sock {
bool zc;
/* Protects multiple processes in the control path */
struct mutex mutex;
+ /* Mutual exclusion of NAPI TX thread and sendmsg error paths
+ * in the SKB destructor callback.
+ */
+ spinlock_t tx_completion_lock;
u64 rx_dropped;
};
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 15aca73805fc..7d220cbd09b6 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -199,8 +199,11 @@ static void xsk_destruct_skb(struct sk_buff *skb)
{
u64 addr = (u64)(long)skb_shinfo(skb)->destructor_arg;
struct xdp_sock *xs = xdp_sk(skb->sk);
+ unsigned long flags;
+ spin_lock_irqsave(&xs->tx_completion_lock, flags);
WARN_ON_ONCE(xskq_produce_addr(xs->umem->cq, addr));
+ spin_unlock_irqrestore(&xs->tx_completion_lock, flags);
sock_wfree(skb);
}
@@ -755,6 +758,7 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
xs = xdp_sk(sk);
mutex_init(&xs->mutex);
+ spin_lock_init(&xs->tx_completion_lock);
local_bh_disable();
sock_prot_inuse_add(net, &xsk_proto, 1);