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 <l...@amacapital.net>
---
arch/x86/include/asm/mmu_context.h | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/mmu_context.h
b/arch/x86/include/asm/mmu_context.h
index 166af2a8e865..23697f74b372 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -53,7 +53,18 @@ 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 that prev->context.ldt doesn't match
+ * the LDT register. This can happen if leave_mm(prev)
+ * was called and then modify_ldt changed
+ * prev->context.ldt but suppressed an IPI to this CPU.
+ * In this case, prev->context.ldt != NULL, because we
+ * never free an LDT while the mm still exists. That
+ * means that next->context.ldt != prev->context.ldt,
+ * because mms never share an LDT.
+ */
if (unlikely(prev->context.ldt != next->context.ldt))
load_LDT_nolock(&next->context);
}
--
1.9.3
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