[PATCH] get 2% or more performance improved by reducing spin_lock race

[Qinchuanyu]

the wake_up_process func is included by spin_lock/unlock in vhost_work_queue,
 but it could be done outside the spin_lock. 
I have test it with kernel 3.0.27 and guest suse11-sp2 using iperf, the num as 
below.
                 orignal                   modified
thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
1           9.59         28.82   |      9.59        27.49
8            9.61        32.92   |      9.62        26.77
64            9.58        46.48  |     9.55        38.99
256            9.6        63.7   |      9.6         52.59

Signed-off-by: Chuanyu Qin <qinchua...@huawei.com>
---
 drivers/vhost/vhost.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 94dbd25..8bee109 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -146,9 +146,10 @@ static inline void vhost_work_queue(struct vhost_dev *dev,
        if (list_empty(&work->node)) {
                list_add_tail(&work->node, &dev->work_list);
                work->queue_seq++;
+               spin_unlock_irqrestore(&dev->work_lock, flags);
                wake_up_process(dev->worker);
-       }
-       spin_unlock_irqrestore(&dev->work_lock, flags);
+       } else
+               spin_unlock_irqrestore(&dev->work_lock, flags);
 }
 
 void vhost_poll_queue(struct vhost_poll *poll)
-- 
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