Subject: BUG: commit "ARM: Remove __ARCH_WANT_INTERRUPTS_ON_CTXSW on pre-ARMv6 CPUs" breaks armv5 with CONFIG_PREEMPT

From: [email-masked] (Catalin Marinas)
Date: Thu, 20 Jun 2013 12:12:55 +0100

On Thu, Jun 20, 2013 at 11:28:56AM +0100, Catalin Marinas wrote:
> We may need to place the preempt disable/enable at a higher level in the
> scheduler. My theory is that we have a context switch from prev to next.
> We get preempted just before finish_arch_post_lock_switch(), so the MMU
> hasn't been switched yet. The new switch during preemption happens to a
> thread with the same next mm, so the scheduler no longer switch_mm() and
> the TIF_SWITCH_MM isn't set for the new thread.
> 
> I'll come back with another patch shortly.

Here's another attempt (as before, only compile-tested):

--------------------8<-------------------------------

>>From 02af0f498a9c4b80c196d52c44ab7700fab0f6db Mon Sep 17 00:00:00 2001
From: Catalin Marinas <[email-masked]>
Date: Thu, 20 Jun 2013 11:59:38 +0100
Subject: [PATCH] arm: Fix deferred mm switch on VIVT processors

As of commit b9d4d42ad9 (ARM: Remove __ARCH_WANT_INTERRUPTS_ON_CTXSW on
pre-ARMv6 CPUs), the mm switching on VIVT processors is done in the
finish_arch_post_lock_switch() function to avoid whole cache flushing
with interrupts disabled. The need for deferred mm switch is stored as a
thread flag (TIF_SWITCH_MM). However, with preemption enabled, we can
have another thread switch before finish_arch_post_lock_switch(). If the
new thread has the same mm as the previous 'next' thread, the scheduler
will not call switch_mm() and the TIF_SWITCH_MM flag won't be set for
the new thread.

This patch moves the switch pending flag to the mm_context_t structure
since this is specific to the mm rather than thread.

Signed-off-by: Catalin Marinas <[email-masked]>
Reported-by: Marc Kleine-Budde <[email-masked]>
---
 arch/arm/include/asm/mmu.h         | 2 ++
 arch/arm/include/asm/mmu_context.h | 8 +++++---
 arch/arm/include/asm/thread_info.h | 1 -
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm/include/asm/mmu.h b/arch/arm/include/asm/mmu.h
index e3d5554..d1b4998 100644
--- a/arch/arm/include/asm/mmu.h
+++ b/arch/arm/include/asm/mmu.h
@@ -6,6 +6,8 @@
 typedef struct {
 #ifdef CONFIG_CPU_HAS_ASID
 	atomic64_t	id;
+#else
+	int		switch_pending;
 #endif
 	unsigned int	vmalloc_seq;
 } mm_context_t;
diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h
index a7b85e0..4309ae5 100644
--- a/arch/arm/include/asm/mmu_context.h
+++ b/arch/arm/include/asm/mmu_context.h
@@ -47,7 +47,7 @@ static inline void check_and_switch_context(struct mm_struct *mm,
 		 * on non-ASID CPUs, the old mm will remain valid until the
 		 * finish_arch_post_lock_switch() call.
 		 */
-		set_ti_thread_flag(task_thread_info(tsk), TIF_SWITCH_MM);
+		mm->context.switch_pending = 1;
 	else
 		cpu_switch_mm(mm->pgd, mm);
 }
@@ -56,9 +56,11 @@ static inline void check_and_switch_context(struct mm_struct *mm,
 	finish_arch_post_lock_switch
 static inline void finish_arch_post_lock_switch(void)
 {
-	if (test_and_clear_thread_flag(TIF_SWITCH_MM)) {
-		struct mm_struct *mm = current->mm;
+	struct mm_struct *mm = current->mm;
+
+	if (mm->context.switch_pending) {
 		cpu_switch_mm(mm->pgd, mm);
+		mm->context.switch_pending = 0;
 	}
 }
 
diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 1995d1a..f00b569 100644
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -156,7 +156,6 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
 #define TIF_USING_IWMMXT	17
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
 #define TIF_RESTORE_SIGMASK	20
-#define TIF_SWITCH_MM		22	/* deferred switch_mm */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)