From 126fca643e3f6ee6e26533366f77acdd9e07d2ec Mon Sep 17 00:00:00 2001 From: Hariprasad Shenai Date: Tue, 8 Dec 2015 10:09:14 +0530 Subject: [PATCH] cxgb4: prevent simultaneous execution of service_ofldq() Change mutual exclusion mechanism to prevent multiple threads of execution from running in service_ofldq() at the same time. The old mechanism used an implicit guard on the down-call path and none on the restart path and wasn't working. This checking makes the mechanism explicit and is much easier to understand as a result. Based on original work by Casey Leedom Signed-off-by: Hariprasad Shenai Signed-off-by: David S. Miller --- drivers/net/ethernet/chelsio/cxgb4/cxgb4.h | 1 + drivers/net/ethernet/chelsio/cxgb4/sge.c | 56 +++++++++++++++++++--- 2 files changed, 51 insertions(+), 6 deletions(-) diff --git a/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h b/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h index 55a47de544ea..d4076e8e2e0b 100644 --- a/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h +++ b/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h @@ -618,6 +618,7 @@ struct sge_ofld_txq { /* state for an SGE offload Tx queue */ struct adapter *adap; struct sk_buff_head sendq; /* list of backpressured packets */ struct tasklet_struct qresume_tsk; /* restarts the queue */ + bool service_ofldq_running; /* service_ofldq() is processing sendq */ u8 full; /* the Tx ring is full */ unsigned long mapping_err; /* # of I/O MMU packet mapping errors */ } ____cacheline_aligned_in_smp; diff --git a/drivers/net/ethernet/chelsio/cxgb4/sge.c b/drivers/net/ethernet/chelsio/cxgb4/sge.c index 1076c35a8bfd..e1ff7d8d64eb 100644 --- a/drivers/net/ethernet/chelsio/cxgb4/sge.c +++ b/drivers/net/ethernet/chelsio/cxgb4/sge.c @@ -1542,11 +1542,22 @@ static void ofldtxq_stop(struct sge_ofld_txq *q, struct sk_buff *skb) } /** - * service_ofldq - restart a suspended offload queue + * service_ofldq - service/restart a suspended offload queue * @q: the offload queue * - * Services an offload Tx queue by moving packets from its packet queue - * to the HW Tx ring. The function starts and ends with the queue locked. + * Services an offload Tx queue by moving packets from its Pending Send + * Queue to the Hardware TX ring. The function starts and ends with the + * Send Queue locked, but drops the lock while putting the skb at the + * head of the Send Queue onto the Hardware TX Ring. Dropping the lock + * allows more skbs to be added to the Send Queue by other threads. + * The packet being processed at the head of the Pending Send Queue is + * left on the queue in case we experience DMA Mapping errors, etc. + * and need to give up and restart later. + * + * service_ofldq() can be thought of as a task which opportunistically + * uses other threads execution contexts. We use the Offload Queue + * boolean "service_ofldq_running" to make sure that only one instance + * is ever running at a time ... */ static void service_ofldq(struct sge_ofld_txq *q) { @@ -1556,10 +1567,23 @@ static void service_ofldq(struct sge_ofld_txq *q) unsigned int written = 0; unsigned int flits, ndesc; + /* If another thread is currently in service_ofldq() processing the + * Pending Send Queue then there's nothing to do. Otherwise, flag + * that we're doing the work and continue. Examining/modifying + * the Offload Queue boolean "service_ofldq_running" must be done + * while holding the Pending Send Queue Lock. + */ + if (q->service_ofldq_running) + return; + q->service_ofldq_running = true; + while ((skb = skb_peek(&q->sendq)) != NULL && !q->full) { - /* - * We drop the lock but leave skb on sendq, thus retaining - * exclusive access to the state of the queue. + /* We drop the lock while we're working with the skb at the + * head of the Pending Send Queue. This allows more skbs to + * be added to the Pending Send Queue while we're working on + * this one. We don't need to lock to guard the TX Ring + * updates because only one thread of execution is ever + * allowed into service_ofldq() at a time. */ spin_unlock(&q->sendq.lock); @@ -1604,6 +1628,11 @@ static void service_ofldq(struct sge_ofld_txq *q) written = 0; } + /* Reacquire the Pending Send Queue Lock so we can unlink the + * skb we've just successfully transferred to the TX Ring and + * loop for the next skb which may be at the head of the + * Pending Send Queue. + */ spin_lock(&q->sendq.lock); __skb_unlink(skb, &q->sendq); if (is_ofld_imm(skb)) @@ -1611,6 +1640,11 @@ static void service_ofldq(struct sge_ofld_txq *q) } if (likely(written)) ring_tx_db(q->adap, &q->q, written); + + /*Indicate that no thread is processing the Pending Send Queue + * currently. + */ + q->service_ofldq_running = false; } /** @@ -1624,9 +1658,19 @@ static int ofld_xmit(struct sge_ofld_txq *q, struct sk_buff *skb) { skb->priority = calc_tx_flits_ofld(skb); /* save for restart */ spin_lock(&q->sendq.lock); + + /* Queue the new skb onto the Offload Queue's Pending Send Queue. If + * that results in this new skb being the only one on the queue, start + * servicing it. If there are other skbs already on the list, then + * either the queue is currently being processed or it's been stopped + * for some reason and it'll be restarted at a later time. Restart + * paths are triggered by events like experiencing a DMA Mapping Error + * or filling the Hardware TX Ring. + */ __skb_queue_tail(&q->sendq, skb); if (q->sendq.qlen == 1) service_ofldq(q); + spin_unlock(&q->sendq.lock); return NET_XMIT_SUCCESS; } -- 2.20.1