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 <leedom@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h b/drivers/net/ethernet/chelsio/cxgb4/cxgb4.h
index 55a47de..d4076e8 100644 (file)
--- 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 1076c35..e1ff7d8 100644 (file)
--- 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;
 }