IB/qib: Optimize locking for get_txreq()

The current code locks the QP s_lock, followed by the pending_lock, I
guess to to protect against the allocate failing.

This patch only locks the pending_lock, assuming that the empty case
is an exeception, in which case the pending_lock is dropped, and the
original code is executed.  This will save a lock of s_lock in the
normal case.

The observation is that the sdma descriptors will deplete at twice the
rate of txreq's, so this should be rare.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

1 files changed, 33 insertions, 10 deletions

diff --git a/drivers/infiniband/hw/qib/qib_verbs.c b/drivers/infiniband/hw/qib/qib_verbs.c
index a894762da462..7b6c3bffa9d9 100644
--- a/drivers/infiniband/hw/qib/qib_verbs.c
+++ b/drivers/infiniband/hw/qib/qib_verbs.c
@@ -913,8 +913,8 @@ static void copy_io(u32 __iomem *piobuf, struct qib_sge_state *ss,
                 __raw_writel(last, piobuf);
 }
 
-static struct qib_verbs_txreq *get_txreq(struct qib_ibdev *dev,
-                                         struct qib_qp *qp, int *retp)
+static noinline struct qib_verbs_txreq *__get_txreq(struct qib_ibdev *dev,
+                                           struct qib_qp *qp)
 {
         struct qib_verbs_txreq *tx;
         unsigned long flags;
@@ -926,8 +926,9 @@ static struct qib_verbs_txreq *get_txreq(struct qib_ibdev *dev,
                 struct list_head *l = dev->txreq_free.next;
 
                 list_del(l);
+                spin_unlock(&dev->pending_lock);
+                spin_unlock_irqrestore(&qp->s_lock, flags);
                 tx = list_entry(l, struct qib_verbs_txreq, txreq.list);
-                *retp = 0;
         } else {
                 if (ib_qib_state_ops[qp->state] & QIB_PROCESS_RECV_OK &&
                     list_empty(&qp->iowait)) {
@@ -935,14 +936,33 @@ static struct qib_verbs_txreq *get_txreq(struct qib_ibdev *dev,
                         qp->s_flags |= QIB_S_WAIT_TX;
                         list_add_tail(&qp->iowait, &dev->txwait);
                 }
-                tx = NULL;
                 qp->s_flags &= ~QIB_S_BUSY;
-                *retp = -EBUSY;
+                spin_unlock(&dev->pending_lock);
+                spin_unlock_irqrestore(&qp->s_lock, flags);
+                tx = ERR_PTR(-EBUSY);
         }
+        return tx;
+}
 
-        spin_unlock(&dev->pending_lock);
-        spin_unlock_irqrestore(&qp->s_lock, flags);
+static inline struct qib_verbs_txreq *get_txreq(struct qib_ibdev *dev,
+                                         struct qib_qp *qp)
+{
+        struct qib_verbs_txreq *tx;
+        unsigned long flags;
 
+        spin_lock_irqsave(&dev->pending_lock, flags);
+        /* assume the list non empty */
+        if (likely(!list_empty(&dev->txreq_free))) {
+                struct list_head *l = dev->txreq_free.next;
+
+                list_del(l);
+                spin_unlock_irqrestore(&dev->pending_lock, flags);
+                tx = list_entry(l, struct qib_verbs_txreq, txreq.list);
+        } else {
+                /* call slow path to get the extra lock */
+                spin_unlock_irqrestore(&dev->pending_lock, flags);
+                tx =  __get_txreq(dev, qp);
+        }
         return tx;
 }
 
@@ -1122,9 +1142,9 @@ static int qib_verbs_send_dma(struct qib_qp *qp, struct qib_ib_header *hdr,
                 goto bail;
         }
 
-        tx = get_txreq(dev, qp, &ret);
-        if (!tx)
-                goto bail;
+        tx = get_txreq(dev, qp);
+        if (IS_ERR(tx))
+                goto bail_tx;
 
         control = dd->f_setpbc_control(ppd, plen, qp->s_srate,
                                        be16_to_cpu(hdr->lrh[0]) >> 12);
@@ -1195,6 +1215,9 @@ unaligned:
         ibp->n_unaligned++;
 bail:
         return ret;
+bail_tx:
+        ret = PTR_ERR(tx);
+        goto bail;
 }
 
 /*