Merge branch 'next' into for-linus

Prepare first round of input updates for 3.20.

1 files changed, 97 insertions, 22 deletions

diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 57f0ec962d2c..fcd41a166405 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -261,37 +261,74 @@ static struct tracer function_trace __tracer_data =
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE
-static int update_count(void **data)
+static void update_traceon_count(void **data, bool on)
 {
-        unsigned long *count = (long *)data;
+        long *count = (long *)data;
+        long old_count = *count;
 
-        if (!*count)
-                return 0;
+        /*
+         * Tracing gets disabled (or enabled) once per count.
+         * This function can be called at the same time on multiple CPUs.
+         * It is fine if both disable (or enable) tracing, as disabling
+         * (or enabling) the second time doesn't do anything as the
+         * state of the tracer is already disabled (or enabled).
+         * What needs to be synchronized in this case is that the count
+         * only gets decremented once, even if the tracer is disabled
+         * (or enabled) twice, as the second one is really a nop.
+         *
+         * The memory barriers guarantee that we only decrement the
+         * counter once. First the count is read to a local variable
+         * and a read barrier is used to make sure that it is loaded
+         * before checking if the tracer is in the state we want.
+         * If the tracer is not in the state we want, then the count
+         * is guaranteed to be the old count.
+         *
+         * Next the tracer is set to the state we want (disabled or enabled)
+         * then a write memory barrier is used to make sure that
+         * the new state is visible before changing the counter by
+         * one minus the old counter. This guarantees that another CPU
+         * executing this code will see the new state before seeing
+         * the new counter value, and would not do anything if the new
+         * counter is seen.
+         *
+         * Note, there is no synchronization between this and a user
+         * setting the tracing_on file. But we currently don't care
+         * about that.
+         */
+        if (!old_count)
+                return;
 
-        if (*count != -1)
-                (*count)--;
+        /* Make sure we see count before checking tracing state */
+        smp_rmb();
 
-        return 1;
+        if (on == !!tracing_is_on())
+                return;
+
+        if (on)
+                tracing_on();
+        else
+                tracing_off();
+
+        /* unlimited? */
+        if (old_count == -1)
+                return;
+
+        /* Make sure tracing state is visible before updating count */
+        smp_wmb();
+
+        *count = old_count - 1;
 }
 
 static void
 ftrace_traceon_count(unsigned long ip, unsigned long parent_ip, void **data)
 {
-        if (tracing_is_on())
-                return;
-
-        if (update_count(data))
-                tracing_on();
+        update_traceon_count(data, 1);
 }
 
 static void
 ftrace_traceoff_count(unsigned long ip, unsigned long parent_ip, void **data)
 {
-        if (!tracing_is_on())
-                return;
-
-        if (update_count(data))
-                tracing_off();
+        update_traceon_count(data, 0);
 }
 
 static void
@@ -330,11 +367,49 @@ ftrace_stacktrace(unsigned long ip, unsigned long parent_ip, void **data)
 static void
 ftrace_stacktrace_count(unsigned long ip, unsigned long parent_ip, void **data)
 {
-        if (!tracing_is_on())
-                return;
+        long *count = (long *)data;
+        long old_count;
+        long new_count;
 
-        if (update_count(data))
-                trace_dump_stack(STACK_SKIP);
+        /*
+         * Stack traces should only execute the number of times the
+         * user specified in the counter.
+         */
+        do {
+
+                if (!tracing_is_on())
+                        return;
+
+                old_count = *count;
+
+                if (!old_count)
+                        return;
+
+                /* unlimited? */
+                if (old_count == -1) {
+                        trace_dump_stack(STACK_SKIP);
+                        return;
+                }
+
+                new_count = old_count - 1;
+                new_count = cmpxchg(count, old_count, new_count);
+                if (new_count == old_count)
+                        trace_dump_stack(STACK_SKIP);
+
+        } while (new_count != old_count);
+}
+
+static int update_count(void **data)
+{
+        unsigned long *count = (long *)data;
+
+        if (!*count)
+                return 0;
+
+        if (*count != -1)
+                (*count)--;
+
+        return 1;
 }
 
 static void
@@ -361,7 +436,7 @@ ftrace_probe_print(const char *name, struct seq_file *m,
         seq_printf(m, "%ps:%s", (void *)ip, name);
 
         if (count == -1)
-                seq_printf(m, ":unlimited\n");
+                seq_puts(m, ":unlimited\n");
         else
                 seq_printf(m, ":count=%ld\n", count);