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Date: Thu, 19 Mar 2009 10:02:24 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: Steven Rostedt <srostedt@...hat.com>
CC: Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: protect running nmi (V3)
Steven Rostedt wrote:
> On Wed, 2009-03-18 at 16:42 +0800, Lai Jiangshan wrote:
>
>>
>> +static void wait_for_nmi_and_set_mod_flag(void)
>> +{
>> + if (!atomic_cmpxchg(&nmi_running, 0, MOD_CODE_WRITE_FLAG))
>> + rerurn;
>
> You might want to compile check patches before sending. I'll look this
> patch over in detail, and make the necessary fixes.
>
Sorry for it.
(I set CONFIG_DYNAMIC_FTRACE=n temporarily yesterday, ^_^)
Subject: [PATCH] ftrace: protect running nmi (V3)
When I review the sensitive code ftrace_nmi_enter(), I found
the atomic variable nmi_running does protect NMI VS do_ftrace_mod_code(),
but it can not protects NMI(entered nmi) VS NMI(ftrace_nmi_enter()).
cpu#1 | cpu#2 | cpu#3
ftrace_nmi_enter() | do_ftrace_mod_code() |
not modify | |
------------------------|-----------------------|--
executing | set mod_code_write = 1|
executing --|-----------------------|--------------------
executing | | ftrace_nmi_enter()
executing | | do modify
------------------------|-----------------------|-----------------
ftrace_nmi_exit() | |
cpu#3 may be being modified the code which is still being executed on cpu#1,
it will have undefined results and possibly take a GPF, this patch
prevents it occurred.
Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 1d0d7f4..f0c7fad 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -79,11 +79,11 @@ static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
*
* 1) Put the instruction pointer into the IP buffer
* and the new code into the "code" buffer.
- * 2) Set a flag that says we are modifying code
- * 3) Wait for any running NMIs to finish.
- * 4) Write the code
- * 5) clear the flag.
- * 6) Wait for any running NMIs to finish.
+ * 2) Wait for any running NMIs to finish and set a flag that says
+ * we are modifying code, it is done in an atomic operation.
+ * 3) Write the code
+ * 4) clear the flag.
+ * 5) Wait for any running NMIs to finish.
*
* If an NMI is executed, the first thing it does is to call
* "ftrace_nmi_enter". This will check if the flag is set to write
@@ -95,9 +95,9 @@ static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
* are the same as what exists.
*/
+#define MOD_CODE_WRITE_FLAG (1 << 31) /* set when NMI should do the write */
static atomic_t nmi_running = ATOMIC_INIT(0);
static int mod_code_status; /* holds return value of text write */
-static int mod_code_write; /* set when NMI should do the write */
static void *mod_code_ip; /* holds the IP to write to */
static void *mod_code_newcode; /* holds the text to write to the IP */
@@ -114,6 +114,19 @@ int ftrace_arch_read_dyn_info(char *buf, int size)
return r;
}
+static void clear_mod_flag(void)
+{
+ for (;;) {
+ int old = atomic_read(&nmi_running);
+ int new = old & ~MOD_CODE_WRITE_FLAG;
+
+ if (old == new)
+ break;
+
+ old = atomic_cmpxchg(&nmi_running, old, new);
+ }
+}
+
static void ftrace_mod_code(void)
{
/*
@@ -127,27 +140,39 @@ static void ftrace_mod_code(void)
/* if we fail, then kill any new writers */
if (mod_code_status)
- mod_code_write = 0;
+ clear_mod_flag();
}
void ftrace_nmi_enter(void)
{
- atomic_inc(&nmi_running);
- /* Must have nmi_running seen before reading write flag */
- smp_mb();
- if (mod_code_write) {
+ if (atomic_inc_return(&nmi_running) & MOD_CODE_WRITE_FLAG) {
+ smp_rmb();
ftrace_mod_code();
atomic_inc(&nmi_update_count);
}
+ /* Must have previous changes seen before executions */
+ smp_mb();
}
void ftrace_nmi_exit(void)
{
/* Finish all executions before clearing nmi_running */
- smp_wmb();
+ smp_mb();
atomic_dec(&nmi_running);
}
+static void wait_for_nmi_and_set_mod_flag(void)
+{
+ if (!atomic_cmpxchg(&nmi_running, 0, MOD_CODE_WRITE_FLAG))
+ return;
+
+ do {
+ cpu_relax();
+ } while (atomic_cmpxchg(&nmi_running, 0, MOD_CODE_WRITE_FLAG));
+
+ nmi_wait_count++;
+}
+
static void wait_for_nmi(void)
{
if (!atomic_read(&nmi_running))
@@ -167,14 +192,9 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
mod_code_newcode = new_code;
/* The buffers need to be visible before we let NMIs write them */
- smp_wmb();
-
- mod_code_write = 1;
-
- /* Make sure write bit is visible before we wait on NMIs */
smp_mb();
- wait_for_nmi();
+ wait_for_nmi_and_set_mod_flag();
/* Make sure all running NMIs have finished before we write the code */
smp_mb();
@@ -182,13 +202,9 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
ftrace_mod_code();
/* Make sure the write happens before clearing the bit */
- smp_wmb();
-
- mod_code_write = 0;
-
- /* make sure NMIs see the cleared bit */
smp_mb();
+ clear_mod_flag();
wait_for_nmi();
return mod_code_status;
--
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