3.4.15-rt25-rc1 stable review

Dear RT Folks,

This is the RT stable review cycle of patch 3.4.15-rt25-rc1.

Please scream at me if I messed something up. Please test the patches too.

The -rc release will be uploaded to kernel.org and will be deleted when the final release is out. This is just a review release (or release candidate).

The pre-releases will not be pushed to the git repository, only the final release is.

If all goes well, this patch will be converted to the next main release on 10/29/2013.

Enjoy,

-- Steve

To build 3.4.15-rt25-rc1 directly, the following patches should be applied:

http://www.kernel.org/pub/linux/kernel/v3.x/linux-3.4.tar.xz

http://www.kernel.org/pub/linux/kernel/v3.x/patch-3.4.15.xz

http://www.kernel.org/pub/linux/kernel/projects/rt/3.4/patch-3.4.15-rt25-rc1.patch.xz

You can also build from 3.4.15-rt24 by applying the incremental patch:

http://www.kernel.org/pub/linux/kernel/projects/rt/3.4/incr/patch-3.4.15-rt24-rt25-rc1.patch.xz

Changes from 3.4.15-rt24:

---

Steven Rostedt (1):

      Linux 3.4.15-rt25-rc1

Thomas Gleixner (1):
rcu: rcutiny: Prevent RCU stall

----

 kernel/rcutiny_plugin.h |    2 +-
 localversion-rt         |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

From: Thomas Gleixner <[email protected]>

rcu_read_unlock_special() checks in_serving_softirq() and leaves early when true. On RT this is obviously wrong as softirq processing context can be preempted and therefor such a task can be on the gp_tasks list. Leaving early here will leave the task on the list and therefor block RCU processing forever.

This cannot happen on mainline because softirq processing context cannot be preempted and therefor this can never happen at all.

In fact this check looks quite questionable in general. Neither irq context nor softirq processing context in mainline can ever be preempted in mainline so the special unlock case should not ever be invoked in such context. Now the only explanation might be a rcu_read_unlock() being interrupted and therefor leave the rcu nest count at 0 before the special unlock bit has been cleared. That looks fragile. At least it's missing a big fat comment. Paul ????

See mainline commits: ec433f0c5 and 8762705a for further enlightment.

---
 kernel/rcutiny_plugin.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 22ecea0..97e359b 100644
@@ -572,7 +572,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 		rcu_preempt_cpu_qs();
 
 	/* Hardware IRQ handlers cannot block. */
-	if (in_irq() || in_serving_softirq()) {
+	if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) {
 		local_irq_restore(flags);
 		return;
 	}

1.7.10.4

From: Steven Rostedt <[email protected]>

---
 localversion-rt |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index b2111a2..792cb5f 100644
@@ -1 +1 @@
--rt24
+-rt25-rc1

1.7.10.4