linux-kernel - Re: next: Commit 'mm: Prevent __alloc_pages_nodemask() RCU CPU stall ...' causing hang on sparc32 qemu

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Date:   Tue, 29 Nov 2016 17:28:17 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        sparclinux@...r.kernel.org, davem@...emloft.net
Subject: Re: next: Commit 'mm: Prevent __alloc_pages_nodemask() RCU CPU stall
 ...' causing hang on sparc32 qemu

On Tue, Nov 29, 2016 at 01:23:08PM -0800, Guenter Roeck wrote:
> Hi Paul,
> 
> most of my qemu tests for sparc32 targets started to fail in next-20161129.
> The problem is only seen in SMP builds; non-SMP builds are fine.
> Bisect points to commit 2d66cccd73436 ("mm: Prevent __alloc_pages_nodemask()
> RCU CPU stall warnings"); reverting that commit fixes the problem.
> 
> Test scripts are available at:
> 	https://github.com/groeck/linux-build-test/tree/master/rootfs/sparc
> Test results are at:
> 	https://github.com/groeck/linux-build-test/tree/master/rootfs/sparc
> 
> Bisect log is attached.
> 
> Please let me know if there is anything I can do to help tracking down the
> problem.

Apologies!!!  Does the patch below help?

							Thanx, Paul

------------------------------------------------------------------------

commit 97708e737e2a55fed4bdbc005bf05ea909df6b73
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Tue Nov 29 11:06:05 2016 -0800

    rcu: Allow boot-time use of cond_resched_rcu_qs()
    
    The cond_resched_rcu_qs() macro is used to force RCU quiescent states into
    long-running in-kernel loops.  However, some of these loops can execute
    during early boot when interrupts are disabled, and during which time
    it is therefore illegal to enter the scheduler.  This commit therefore
    makes cond_resched_rcu_qs() be a no-op during early boot.
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 525ca34603b7..b6944cc19a07 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -423,7 +423,7 @@ extern struct srcu_struct tasks_rcu_exit_srcu;
  */
 #define cond_resched_rcu_qs() \
 do { \
-	if (!cond_resched()) \
+	if (!is_idle_task(current) && !cond_resched()) \
 		rcu_note_voluntary_context_switch(current); \
 } while (0)
 
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 7232d199a81c..20f5990deeee 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -228,6 +228,7 @@ static inline void exit_rcu(void)
 extern int rcu_scheduler_active __read_mostly;
 void rcu_scheduler_starting(void);
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+#define rcu_scheduler_active false
 static inline void rcu_scheduler_starting(void)
 {
 }