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Subject[PATCH 2/2] ftrace: fast path for do_ftrace_mod_code()
Lai Jiangshan wrote:
>
> Subject: [PATCH 1/2] ftrace: protect running nmi (V2)
>


Subject: [PATCH 2/2] ftrace: fast path for do_ftrace_mod_code()

commit 90c7ac49aa819feb9433b5310089fca6399881c0
adds a fast path to prevent NMI lockup.

But the previous patch "protect executing nmi" changes
do_ftrace_mod_code()'s implementation, we still need fix to
prevent NMI lockup by adding a fast path.

A difference between this fix and 90c7ac49aa819feb9433b5310089fca6399881c0
is that: We kill any new writers in spite of probe_kernel_write()
success or fail, not only when probe_kernel_write() fail.
(When probe_kernel_write() success, new writers do not need to do
it again.)

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 699a1c0..61cb520 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -98,6 +98,7 @@ static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
#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_no_write = 1; /* set when NMI not need 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 */

@@ -124,14 +125,19 @@ static void ftrace_mod_code(void)
*/
mod_code_status = probe_kernel_write(mod_code_ip, mod_code_newcode,
MCOUNT_INSN_SIZE);
+
+ smb_wmb();
+ mod_code_no_write = 1;
}

void ftrace_nmi_enter(void)
{
if (atomic_inc_return(&nmi_running) & MOD_CODE_WRITE_FLAG) {
smp_rmb();
- ftrace_mod_code();
- atomic_inc(&nmi_update_count);
+ if (!mod_code_no_write) {
+ ftrace_mod_code();
+ atomic_inc(&nmi_update_count);
+ }
}
/* Must have previous changes seen before executions */
smp_mb();
@@ -161,6 +167,7 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
{
mod_code_ip = (void *)ip;
mod_code_newcode = new_code;
+ mod_code_no_write = 0;

/*
* The previous variables need to be visible before NMIs sees
@@ -173,7 +180,8 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
/* Make sure all running NMIs have finished before we write the code */
smp_mb();

- ftrace_mod_code();
+ if (!mod_code_no_write)
+ ftrace_mod_code();

/* Make sure the write happens before clearing the bit */
smp_mb();




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