1 files changed, 145 insertions, 1 deletions

diff --git a/arch/sh/kernel/ftrace.c b/arch/sh/kernel/ftrace.c
index 2c48e267256e..b6f41c109beb 100644
--- a/arch/sh/kernel/ftrace.c
+++ b/arch/sh/kernel/ftrace.c
@@ -62,6 +62,150 @@ static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
         return ftrace_replaced_code;
 }
 
+/*
+ * Modifying code must take extra care. On an SMP machine, if
+ * the code being modified is also being executed on another CPU
+ * that CPU will have undefined results and possibly take a GPF.
+ * We use kstop_machine to stop other CPUS from exectuing code.
+ * But this does not stop NMIs from happening. We still need
+ * to protect against that. We separate out the modification of
+ * the code to take care of this.
+ *
+ * Two buffers are added: An IP buffer and a "code" buffer.
+ *
+ * 1) Put the instruction pointer into the IP buffer
+ *    and the new code into the "code" buffer.
+ * 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
+ * and if it is, it will write what is in the IP and "code" buffers.
+ *
+ * The trick is, it does not matter if everyone is writing the same
+ * content to the code location. Also, if a CPU is executing code
+ * it is OK to write to that code location if the contents being written
+ * 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 void *mod_code_ip;               /* holds the IP to write to */
+static void *mod_code_newcode;          /* holds the text to write to the IP */
+
+static unsigned nmi_wait_count;
+static atomic_t nmi_update_count = ATOMIC_INIT(0);
+
+int ftrace_arch_read_dyn_info(char *buf, int size)
+{
+        int r;
+
+        r = snprintf(buf, size, "%u %u",
+                     nmi_wait_count,
+                     atomic_read(&nmi_update_count));
+        return r;
+}
+
+static void clear_mod_flag(void)
+{
+        int old = atomic_read(&nmi_running);
+
+        for (;;) {
+                int new = old & ~MOD_CODE_WRITE_FLAG;
+
+                if (old == new)
+                        break;
+
+                old = atomic_cmpxchg(&nmi_running, old, new);
+        }
+}
+
+static void ftrace_mod_code(void)
+{
+        /*
+         * Yes, more than one CPU process can be writing to mod_code_status.
+         *    (and the code itself)
+         * But if one were to fail, then they all should, and if one were
+         * to succeed, then they all should.
+         */
+        mod_code_status = probe_kernel_write(mod_code_ip, mod_code_newcode,
+                                             MCOUNT_INSN_SIZE);
+
+        /* if we fail, then kill any new writers */
+        if (mod_code_status)
+                clear_mod_flag();
+}
+
+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);
+        }
+        /* Must have previous changes seen before executions */
+        smp_mb();
+}
+
+void ftrace_nmi_exit(void)
+{
+        /* Finish all executions before clearing nmi_running */
+        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))
+                return;
+
+        do {
+                cpu_relax();
+        } while (atomic_read(&nmi_running));
+
+        nmi_wait_count++;
+}
+
+static int
+do_ftrace_mod_code(unsigned long ip, void *new_code)
+{
+        mod_code_ip = (void *)ip;
+        mod_code_newcode = new_code;
+
+        /* The buffers need to be visible before we let NMIs write them */
+        smp_mb();
+
+        wait_for_nmi_and_set_mod_flag();
+
+        /* Make sure all running NMIs have finished before we write the code */
+        smp_mb();
+
+        ftrace_mod_code();
+
+        /* Make sure the write happens before clearing the bit */
+        smp_mb();
+
+        clear_mod_flag();
+        wait_for_nmi();
+
+        return mod_code_status;
+}
+
 static int ftrace_modify_code(unsigned long ip, unsigned char *old_code,
                        unsigned char *new_code)
 {
@@ -86,7 +230,7 @@ static int ftrace_modify_code(unsigned long ip, unsigned char *old_code,
                 return -EINVAL;
 
         /* replace the text with the new text */
-        if (probe_kernel_write((void *)ip, new_code, MCOUNT_INSN_SIZE))
+        if (do_ftrace_mod_code(ip, new_code))
                 return -EPERM;
 
         flush_icache_range(ip, ip + MCOUNT_INSN_SIZE);