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authorAndy Lutomirski <luto@amacapital.net>2014-10-24 15:58:09 -0700
committerIngo Molnar <mingo@kernel.org>2015-02-04 12:10:43 +0100
commit22c4bd9fa921c2b1b3f2420d7b9dabbe982f3059 (patch)
treed3bf2be30a8db9eda07060a1f5d84d17250a4a59 /arch
parent1e02ce4cccdcb9688386e5b8d2c9fa4660b45389 (diff)
downloadlinux-22c4bd9fa921c2b1b3f2420d7b9dabbe982f3059.tar.bz2
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/include/asm/mmu_context.h14
1 files changed, 8 insertions, 6 deletions
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 4b75d591eb5e..52c18359f1dc 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);