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Date:   Thu, 30 Aug 2018 10:32:13 -0700
From:   Nadav Amit <>
To:     Thomas Gleixner <>
CC:     <>, Ingo Molnar <>,
        <>, Arnd Bergmann <>,
        Dave Hansen <>,
        Nadav Amit <>,
        Andy Lutomirski <>,
        Masami Hiramatsu <>,
        Kees Cook <>,
        Dave Hansen <>
Subject: [PATCH 1/6] x86/alternatives: clarify text_mutex use in text_poke

text_mutex is expected to be held before text_poke() is called, but we
cannot add a lockdep assertion since kgdb does not take it, and instead
*supposedly* ensures the lock is not taken and will not be acquired by
any other core while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely clear
that this would be the case if gdb_do_roundup is zero.

Add a comment to clarify this behavior.

Cc: Andy Lutomirski <>
Cc: Masami Hiramatsu <>
Cc: Kees Cook <>
Cc: Dave Hansen <>
Suggested-by: Peter Zijlstra <>
Signed-off-by: Nadav Amit <>
 arch/x86/kernel/alternative.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..d0c0d8b724e1 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -685,7 +685,10 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  * in a way that permits an atomic write. It also makes sure we fit on a single
  * page.
- * Note: Must be called under text_mutex.
+ * Context: Must be called under text_mutex. kgdb is an exception: it does not
+ *	    hold the mutex, as it *supposedly* ensures that no other core is
+ *	    holding the mutex and ensures that none of them will acquire the
+ *	    mutex while the code runs.
 void *text_poke(void *addr, const void *opcode, size_t len)

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