Patch "ARM: Hide finish_arch_post_lock_switch() from modules" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ARM: Hide finish_arch_post_lock_switch() from modules

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm-hide-finish_arch_post_lock_switch-from-modules.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Steven Rostedt <[email protected]>
Date: Fri, 13 May 2016 15:30:13 +0200
Subject: ARM: Hide finish_arch_post_lock_switch() from modules

From: Steven Rostedt <[email protected]>

commit ef0491ea17f8019821c7e9c8e801184ecf17f85a upstream.

The introduction of switch_mm_irqs_off() brought back an old bug regarding the use of preempt_enable_no_resched:

As part of:

62b94a08da1b ("sched/preempt: Take away preempt_enable_no_resched() from modules")

the definition of preempt_enable_no_resched() is only available in built-in code, not in loadable modules, so we can't generally use it from header files.

However, the ARM version of finish_arch_post_lock_switch() calls preempt_enable_no_resched() and is defined as a static inline function in asm/mmu_context.h. This in turn means we cannot include asm/mmu_context.h from modules.

With today's tip tree, asm/mmu_context.h gets included from linux/mmu_context.h, which is normally the exact pattern one would expect, but unfortunately, linux/mmu_context.h can be included from the vhost driver that is a loadable module, now causing this compile time error with modular configs:

In file included from ../include/linux/mmu_context.h:4:0,

                   from ../drivers/vhost/vhost.c:18:
  ../arch/arm/include/asm/mmu_context.h: In function 'finish_arch_post_lock_switch':
  ../arch/arm/include/asm/mmu_context.h:88:3: error: implicit declaration of function 'preempt_enable_no_resched' [-Werror=implicit-function-declaration]
     preempt_enable_no_resched();

Andy already tried to fix the bug by including linux/preempt.h from asm/mmu_context.h, but that didn't help. Arnd suggested reordering the header files, which wasn't popular, so let's use this workaround instead:

The finish_arch_post_lock_switch() definition is now also hidden inside of #ifdef MODULE, so we don't see anything referencing preempt_enable_no_resched() from a header file. I've built a few hundred randconfig kernels with this, and did not see any new problems.

---
 arch/arm/include/asm/mmu_context.h |    2 ++
 1 file changed, 2 insertions(+)

@@ -61,6 +61,7 @@ static inline void check_and_switch_cont
 		cpu_switch_mm(mm->pgd, mm);
 }
 
+#ifndef MODULE
 #define finish_arch_post_lock_switch \
 	finish_arch_post_lock_switch
 static inline void finish_arch_post_lock_switch(void)

@@ -82,6 +83,7 @@ static inline void finish_arch_post_lock
 		preempt_enable_no_resched();
 	}
 }
+#endif /* !MODULE */
 
 #endif	/* CONFIG_MMU */
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/arm-hide-finish_arch_post_lock_switch-from-modules.patch