x86: Add a comment clarifying LDT context switching

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@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Vince Weaver <vince@deater.net>
Cc: "hillf.zj" <hillf.zj@alibaba-inc.com>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/0db86397f968996fb772c443c251415b0b430ddd.1414190806.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 4b75d591eb5ed1e4757ef8b658d08a2ad02a84cd..52c18359f1dcaf68e093007e17a30a8f71acee37 100644 (file)
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -55,12 +55,14 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
                /*
                 * 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.
+                * 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);