[PATCH v3 0/2] arm64: stop using kstop_machine for ftrace

[PATCH v3 0/2] arm64: stop using kstop_machine for ftrace

[Li Bin]

v2:
Based on the comments from Will and Steve,
1. Modify the commit message
2. Fix the misleading comments for ftrace_modify_code

v3:
Modify the comments again based on the comment from Steve.

Link: https://lkml.org/lkml/2015/12/3/422

Li Bin (2):
  arm64: ftrace: stop using kstop_machine to enable/disable tracing
  arm64: ftrace: fix the comments for ftrace_modify_code

 arch/arm64/kernel/ftrace.c |   16 ++++++++++------
 1 files changed, 10 insertions(+), 6 deletions(-)

[PATCH v3 1/2] arm64: ftrace: stop using kstop_machine to enable/disable tracing

[Li Bin]

For ftrace on arm64, kstop_machine which is hugely disruptive
to a running system is not needed to convert nops to ftrace calls
or back, because that to be modified instrucions, that NOP, B or BL,
are all safe instructions which called "concurrent modification
and execution of instructions", that can be executed by one
thread of execution as they are being modified by another thread
of execution without requiring explicit synchronization.

Signed-off-by: Li Bin <huawei.libin@huawei.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
---
 arch/arm64/kernel/ftrace.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index c851be7..9669b33 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -93,6 +93,11 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 	return ftrace_modify_code(pc, old, new, true);
 }
 
+void arch_ftrace_update_code(int command)
+{
+	ftrace_modify_all_code(command);
+}
+
 int __init ftrace_dyn_arch_init(void)
 {
 	return 0;
-- 
1.7.1

[PATCH v3 2/2] arm64: ftrace: fix the comments for ftrace_modify_code

[Li Bin]

There is no need to worry about module and __init text disappearing
case, because that ftrace has a module notifier that is called when
a module is being unloaded and before the text goes away and this
code grabs the ftrace_lock mutex and removes the module functions
from the ftrace list, such that it will no longer do any
modifications to that module's text, the update to make functions
be traced or not is done under the ftrace_lock mutex as well.
And by now, __init section codes should not been modified
by ftrace, because it is black listed in recordmcount.c and
ignored by ftrace.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Li Bin <huawei.libin@huawei.com>
---
 arch/arm64/kernel/ftrace.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 9669b33..8f7005b 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -29,12 +29,11 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new,
 
 	/*
 	 * Note:
-	 * Due to modules and __init, code can disappear and change,
-	 * we need to protect against faulting as well as code changing.
-	 * We do this by aarch64_insn_*() which use the probe_kernel_*().
-	 *
-	 * No lock is held here because all the modifications are run
-	 * through stop_machine().
+	 * We are paranoid about modifying text, as if a bug were to happen, it
+	 * could cause us to read or write to someplace that could cause harm.
+	 * Carefully read and modify the code with aarch64_insn_*() which uses
+	 * probe_kernel_*(), and make sure what we read is what we expected it
+	 * to be before modifying it.
 	 */
 	if (validate) {
 		if (aarch64_insn_read((void *)pc, &replaced))
-- 
1.7.1

[PATCH v3 0/2] arm64: stop using kstop_machine for ftrace

[Will Deacon]

On Fri, Dec 04, 2015 at 11:38:38AM +0800, Li Bin wrote:
> v2:
> Based on the comments from Will and Steve,
> 1. Modify the commit message
> 2. Fix the misleading comments for ftrace_modify_code
> 
> v3:
> Modify the comments again based on the comment from Steve.
> 
> Link: https://lkml.org/lkml/2015/12/3/422
> 
> Li Bin (2):
>   arm64: ftrace: stop using kstop_machine to enable/disable tracing
>   arm64: ftrace: fix the comments for ftrace_modify_code

Thanks, I'll queue both of these for 4.5.

Will