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Commit 22c4bd9f authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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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: default avatarAndy Lutomirski <luto@amacapital.net>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 1e02ce4c

arch/x86/include/asm/mmu_context.h

+8 −6
Original line number Diff line number Diff line
@@ -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);