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[PATCH 3/3] x86/mm: Turn off IRQs in switch_mm

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Potential races between switch_mm and TLB-flush or LDT-flush IPIs
could be very messy.  AFAICT the code is currently okay, whether by
accident or by careful design, but enabling PCID will make it
considerably more complicated and will no longer be obviously safe.

Fix it with a bug hammer: run switch_mm with IRQs off.

To avoid a performance hit in the scheduler, we take advantage of
our knowledge that the scheduler already has IRQs disabled when it
calls switch_mm.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/mmu_context.h |  3 +++
 arch/x86/mm/tlb.c                  | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index bb911dd7cd01..396348196aa7 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -118,6 +118,9 @@ static inline void destroy_context(struct mm_struct *mm)
 extern void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		      struct task_struct *tsk);
 
+extern void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
+			       struct task_struct *tsk);
+#define switch_mm_irqs_off switch_mm_irqs_off
 
 #define activate_mm(prev, next)			\
 do {						\
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index c826ea193279..40acd07da324 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -62,6 +62,16 @@ EXPORT_SYMBOL_GPL(leave_mm);
 void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	       struct task_struct *tsk)
 {
+	unsigned long flags;
+
+	local_irq_save(flags);
+	switch_mm_irqs_off(prev, next, tsk);
+	local_irq_restore(flags);
+}
+
+void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
+			struct task_struct *tsk)
+{
 	unsigned cpu = smp_processor_id();
 
 	if (likely(prev != next)) {
-- 
2.5.5

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[PATCH 0/3] Make x86's switch_mm run with IRQs off Andy Lutomirski <luto@kernel.org> - 2016-04-21 20:00 +0200
  [PATCH 2/3] x86/mm: Uninline switch_mm Andy Lutomirski <luto@kernel.org> - 2016-04-21 20:00 +0200
  [PATCH 3/3] x86/mm: Turn off IRQs in switch_mm Andy Lutomirski <luto@kernel.org> - 2016-04-21 20:00 +0200
  [PATCH 1/3] sched: Add switch_mm_irqs_off and use it in the scheduler Andy Lutomirski <luto@kernel.org> - 2016-04-21 20:00 +0200
  Re: [PATCH 0/3] Make x86's switch_mm run with IRQs off Andy Lutomirski <luto@amacapital.net> - 2016-04-21 20:50 +0200

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