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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: 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>
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@ -55,12 +55,14 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
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/*
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* Load the LDT, if the LDT is different.
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*
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* It's possible leave_mm(prev) has been called. If so,
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* then prev->context.ldt could be out of sync with the
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* LDT descriptor or the LDT register. This can only happen
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* if prev->context.ldt is non-null, since we never free
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* an LDT. But LDTs can't be shared across mms, so
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* prev->context.ldt won't be equal to next->context.ldt.
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* It's possible that prev->context.ldt doesn't match
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* the LDT register. This can happen if leave_mm(prev)
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* was called and then modify_ldt changed
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* prev->context.ldt but suppressed an IPI to this CPU.
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* In this case, prev->context.ldt != NULL, because we
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* never free an LDT while the mm still exists. That
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* means that next->context.ldt != prev->context.ldt,
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* because mms never share an LDT.
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*/
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if (unlikely(prev->context.ldt != next->context.ldt))
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load_LDT_nolock(&next->context);
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