git: 66acf7685bcd - main - if_epair: fix race condition on multi-core systems

The branch main has been updated by kp:

URL: https://cgit.FreeBSD.org/src/commit/?id=66acf7685bcd8cf23b6c658a991637238a01859e

commit 66acf7685bcd8cf23b6c658a991637238a01859e
Author:     Michael Gmelin <grembo@FreeBSD.org>
AuthorDate: 2022-03-16 22:08:55 +0000
Commit:     Kristof Provost <kp@FreeBSD.org>
CommitDate: 2022-03-16 22:08:55 +0000

    if_epair: fix race condition on multi-core systems
    
    As an unwanted side effect of the performance improvements in
    24f0bfbad57b9, epair interfaces stop forwarding traffic on higher
    load levels when running on multi-core systems.
    
    This happens due to a race condition in the logic that decides when to
    place work in the task queue(s) responsible for processing the content
    of ring buffers.
    
    In order to fix this, a field named state is added to the epair_queue
    structure. This field is used by the affected functions to signal each
    other that something happened in the underlying ring buffers that might
    require work to be scheduled in task queue(s), replacing the existing
    logic, which relied on checking if ring buffers are empty or not.
    
    epair_menq() does:
      - set BIT_MBUF_QUEUED
      - queue mbuf
      - if testandset BIT_QUEUE_TASK:
          enqueue task
    
    epair_tx_start_deferred() does:
      - swap ring buffers
      - process mbufs
      - clear BIT_QUEUE_TASK
      - if testandclear BIT_MBUF_QUEUED
          enqueue task
    
    PR:             262571
    Reported by:    Johan Hendriks <joh.hendriks@gmail.com>
    MFC after:      3 days
    Differential Revision:  https://reviews.freebsd.org/D34569
---
 sys/net/if_epair.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/sys/net/if_epair.c b/sys/net/if_epair.c
index c39434b31b9f..fbf48c7793ea 100644
--- a/sys/net/if_epair.c
+++ b/sys/net/if_epair.c
@@ -104,11 +104,15 @@ static unsigned int next_index = 0;
 #define	EPAIR_LOCK()			mtx_lock(&epair_n_index_mtx)
 #define	EPAIR_UNLOCK()			mtx_unlock(&epair_n_index_mtx)
 
+#define BIT_QUEUE_TASK		0
+#define BIT_MBUF_QUEUED		1
+
 struct epair_softc;
 struct epair_queue {
 	int			 id;
 	struct buf_ring		*rxring[2];
 	volatile int		 ridx;		/* 0 || 1 */
+	volatile int		 state;		/* taskqueue coordination */
 	struct task		 tx_task;
 	struct epair_softc	*sc;
 };
@@ -177,7 +181,8 @@ epair_tx_start_deferred(void *arg, int pending)
 	} while (!atomic_fcmpset_int(&q->ridx, &ridx, nidx));
 	epair_if_input(sc, q, ridx);
 
-	if (! buf_ring_empty(q->rxring[nidx]))
+	atomic_clear_int(&q->state, (1 << BIT_QUEUE_TASK));
+	if (atomic_testandclear_int(&q->state, BIT_MBUF_QUEUED))
 		taskqueue_enqueue(epair_tasks.tq[q->id], &q->tx_task);
 
 	if_rele(sc->ifp);
@@ -192,7 +197,6 @@ epair_menq(struct mbuf *m, struct epair_softc *osc)
 	short mflags;
 	struct epair_queue *q = NULL;
 	uint32_t bucket;
-	bool was_empty;
 #ifdef RSS
 	struct ether_header *eh;
 #endif
@@ -244,14 +248,14 @@ epair_menq(struct mbuf *m, struct epair_softc *osc)
 #endif
 	q = &osc->queues[bucket];
 
+	atomic_set_int(&q->state, (1 << BIT_MBUF_QUEUED));
 	ridx = atomic_load_int(&q->ridx);
-	was_empty = buf_ring_empty(q->rxring[ridx]);
 	ret = buf_ring_enqueue(q->rxring[ridx], m);
 	if (ret != 0) {
 		/* Ring is full. */
 		if_inc_counter(ifp, IFCOUNTER_OQDROPS, 1);
 		m_freem(m);
-		goto done;
+		return (0);
 	}
 
 	if_inc_counter(ifp, IFCOUNTER_OPACKETS, 1);
@@ -266,8 +270,7 @@ epair_menq(struct mbuf *m, struct epair_softc *osc)
 	/* Someone else received the packet. */
 	if_inc_counter(oifp, IFCOUNTER_IPACKETS, 1);
 
-done:
-	if (was_empty)
+	if (!atomic_testandset_int(&q->state, BIT_QUEUE_TASK))
 		taskqueue_enqueue(epair_tasks.tq[bucket], &q->tx_task);
 
 	return (0);
@@ -552,6 +555,7 @@ epair_clone_create(struct if_clone *ifc, char *name, size_t len, caddr_t params)
 		q->rxring[0] = buf_ring_alloc(RXRSIZE, M_EPAIR, M_WAITOK, NULL);
 		q->rxring[1] = buf_ring_alloc(RXRSIZE, M_EPAIR, M_WAITOK, NULL);
 		q->ridx = 0;
+		q->state = 0;
 		q->sc = sca;
 		NET_TASK_INIT(&q->tx_task, 0, epair_tx_start_deferred, q);
 	}
@@ -574,6 +578,7 @@ epair_clone_create(struct if_clone *ifc, char *name, size_t len, caddr_t params)
 		q->rxring[0] = buf_ring_alloc(RXRSIZE, M_EPAIR, M_WAITOK, NULL);
 		q->rxring[1] = buf_ring_alloc(RXRSIZE, M_EPAIR, M_WAITOK, NULL);
 		q->ridx = 0;
+		q->state = 0;
 		q->sc = scb;
 		NET_TASK_INIT(&q->tx_task, 0, epair_tx_start_deferred, q);
 	}