[3.8.y.z extended stable] Patch "genirq: Remove racy waitqueue_active check" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    genirq: Remove racy waitqueue_active check

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.21.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From d3d64ad6b67e235487022d5a3d8958af4662f7c2 Mon Sep 17 00:00:00 2001
From: Chuansheng Liu <chuansheng.liu at intel.com>
Date: Mon, 24 Feb 2014 11:29:50 +0800
Subject: genirq: Remove racy waitqueue_active check

commit c685689fd24d310343ac33942e9a54a974ae9c43 upstream.

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)
                                             wake_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'd need one smp_mb() before waitqueue_active.that, but removing
the waitqueue_active() check solves it as wel l and it makes
things simple and clear.

Signed-off-by: Chuansheng Liu <chuansheng.liu at intel.com>
Cc: Xiaoming Wang <xiaoming.wang at intel.com>
Link: http://lkml.kernel.org/r/1393212590-32543-1-git-send-email-chuansheng.liu@intel.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 a9302d0..f3feefc 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -801,8 +801,7 @@ 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);
 }

--
1.8.3.2