linux-kernel - [PATCH] ARM: hide finish_arch_post_lock

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Date:	Fri, 13 May 2016 15:30:13 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Ingo Molnar <mingo@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Russell King - ARM Linux <linux@...linux.org.uk>,
	Guenter Roeck <linux@...ck-us.net>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...e.de>,
	Mel Gorman <mgorman@...hsingularity.net>,
	linux-arm-kernel@...ts.infradead.org, Arnd Bergmann <arnd@...db.de>
Subject: [PATCH] ARM: hide finish_arch_post_lock_switch from modules

From: Steven Rostedt <rostedt@...dmis.org>

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:

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. I suggested reordering
the header files, which wasn't popular, but Steve came up with 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.

Fixes: f98db6013c55 ("sched/core: Add switch_mm_irqs_off() and use it in the scheduler")
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Russell King <rmk+kernel@....linux.org.uk>
---
 arch/arm/include/asm/mmu_context.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h
index ed73babc0dc9..3cc14dd8587c 100644
--- a/arch/arm/include/asm/mmu_context.h
+++ b/arch/arm/include/asm/mmu_context.h
@@ -67,6 +67,7 @@ static inline void check_and_switch_context(struct mm_struct *mm,
 		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)
@@ -88,6 +89,7 @@ static inline void finish_arch_post_lock_switch(void)
 		preempt_enable_no_resched();
 	}
 }
+#endif /* !MODULE */

 #endif	/* CONFIG_MMU */

-- 
2.7.0