[patch 3/3] x86/mm: Remove preempt_disable/enable() from __native_flush_tlb()

[Thomas Gleixner <tglx@linutronix.de>]

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The preempt_disable/enable() pair in __native_flush_tlb() was added in
commit 5cf0791da5c1 ("x86/mm: Disable preemption during CR3 read+write") to
protect the UP variant of flush_tlb_mm_range().

That preempt_disable/enable() pair should have been added to the UP variant
of flush_tlb_mm_range() instead.

The UP variant was removed with commit ce4a4e565f52 ("x86/mm: Remove the UP
asm/tlbflush.h code, always use the (formerly) SMP code"), but the
preempt_disable/enable() pair stayed around.

The latest change to __native_flush_tlb() in commit 6fd166aae78c ("x86/mm:
Use/Fix PCID to optimize user/kernel switches") added an access to a per
cpu variable outside the preempt disabled regions which makes no sense at
all. __native_flush_tlb() must always be called with at least preemption
disabled.

Remove the preempt_disable/enable() pair and add a WARN_ON_ONCE() to catch
bad callers independent of the smp_processor_id() debugging.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/include/asm/tlbflush.h |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -345,15 +345,17 @@ static inline void invalidate_user_asid(
  */
 static inline void __native_flush_tlb(void)
 {
-	invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
 	/*
-	 * If current->mm == NULL then we borrow a mm which may change
-	 * during a task switch and therefore we must not be preempted
-	 * while we write CR3 back:
+	 * Preemption or interrupts must be disabled to protect the access
+	 * to the per cpu variable and to prevent being preempted between
+	 * read_cr3() and write_cr3().
 	 */
-	preempt_disable();
+	WARN_ON_ONCE(preemptible());
+
+	invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
+
+	/* If current->mm == NULL then the read_cr3() "borrows" a mm */
 	native_write_cr3(__native_read_cr3());
-	preempt_enable();
 }
 
 /*