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x86: Add a comment clarifying LDT context switching

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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>
This commit is contained in:
Andy Lutomirski 2014-10-24 15:58:09 -07:00 committed by Ingo Molnar
parent 1e02ce4ccc
commit 22c4bd9fa9
1 changed files with 8 additions and 6 deletions
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14
arch/x86/include/asm/mmu_context.h
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@ -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);