linux-kernel - [tip:perf/x86] x86: Add a comment clarifying LDT context switching

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Message-ID: <tip-22c4bd9fa921c2b1b3f2420d7b9dabbe982f3059@git.kernel.org>
Date:	Wed, 4 Feb 2015 06:41:59 -0800
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	hpa@...or.com, hillf.zj@...baba-inc.com, luto@...capital.net,
	linux-kernel@...r.kernel.org, keescook@...omium.org,
	tglx@...utronix.de, peterz@...radead.org,
	torvalds@...ux-foundation.org, aarcange@...hat.com,
	acme@...nel.org, Valdis.Kletnieks@...edu, paulus@...ba.org,
	mingo@...nel.org, vince@...ter.net
Subject: [tip:perf/x86] x86:
  Add a comment clarifying LDT context switching

Commit-ID:  22c4bd9fa921c2b1b3f2420d7b9dabbe982f3059
Gitweb:     http://git.kernel.org/tip/22c4bd9fa921c2b1b3f2420d7b9dabbe982f3059
Author:     Andy Lutomirski <luto@...capital.net>
AuthorDate: Fri, 24 Oct 2014 15:58:09 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 4 Feb 2015 12:10:43 +0100

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@...capital.net>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Vince Weaver <vince@...ter.net>
Cc: "hillf.zj" <hillf.zj@...baba-inc.com>
Cc: Valdis Kletnieks <Valdis.Kletnieks@...edu>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/0db86397f968996fb772c443c251415b0b430ddd.1414190806.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/asm/mmu_context.h | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 4b75d59..52c1835 100644
--- 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);
--
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