[PATCH] genirq: Fix the possible synchronize_irq() wait-forever

[Chuansheng Liu]

We hit one rare case below:
T1 calling disable_irq(), but hanging at synchronize_irq()
always;
The corresponding irq thread is in sleeping state;
And all CPUs are in idle state;

After analysis, we found there is one possible scenerio which
causes T1 is waiting there forever:
CPU0                                       CPU1
 synchronize_irq()
  wait_event()
    spin_lock()
                                           atomic_dec_and_test(&threads_active)
      insert the __wait into queue
    spin_unlock()
                                           if(waitqueue_active)
    atomic_read(&threads_active)
                                             wait_up()

Here after inserted the __wait into queue on CPU0, and before
test if queue is empty on CPU1, there is no barrier, it maybe
cause it is not visible for CPU1 immediately, although CPU0 has
updated the queue list.
It is similar for CPU0 atomic_read() threads_active also.

So we need one smp_mb() before waitqueue_active or something like
that.

Thomas shared one good option that removing waitqueue_active()
judgement directly, it will make things to be simple and clear.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Xiaoming Wang <xiaoming.wang@xxxxxxxxx>
Signed-off-by: Chuansheng Liu <chuansheng.liu@xxxxxxxxx>
---
 kernel/irq/manage.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 481a13c..d3bf660 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -802,8 +802,7 @@ static irqreturn_t irq_thread_fn(struct irq_desc *desc,
 
 static void wake_threads_waitq(struct irq_desc *desc)
 {
-	if (atomic_dec_and_test(&desc->threads_active) &&
-	    waitqueue_active(&desc->wait_for_threads))
+	if (atomic_dec_and_test(&desc->threads_active))
 		wake_up(&desc->wait_for_threads);
 }
 
-- 
1.9.rc0

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