[RFC 3/5] x86: Add a comment clarifying LDT context switching

[Andy Lutomirski]

The code is correct, but only for a rather subtle reason.  This
confused me for quite a while when I read switch_mm, so clarify the
code to avoid confusing other people, too.

TBH, I wouldn't be surprised if this code was only correct by
accident.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/include/asm/mmu_context.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 166af2a8e865..04478103df37 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -53,7 +53,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		/* Stop flush ipis for the previous mm */
 		cpumask_clear_cpu(cpu, mm_cpumask(prev));
 
-		/* Load the LDT, if the LDT is different: */
+		/*
+		 * Load the LDT, if the LDT is different.
+		 *
+		 * It's possible leave_mm(prev) has been called.  If so,
+		 * then prev->context.ldt could be out of sync with the
+		 * LDT descriptor or the LDT register.  This can only happen
+		 * if prev->context.ldt is non-null, since we never free
+		 * an LDT.  But LDTs can't be shared across mms, so
+		 * prev->context.ldt won't be equal to next->context.ldt.
+		 */
 		if (unlikely(prev->context.ldt != next->context.ldt))
 			load_LDT_nolock(&next->context);
 	}
-- 
1.9.3

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