tracing/syscalls: protect thread flag toggling from races

Impact: fix syscall tracer enable/disable race

The current thread flag toggling is racy as shown in the following
scenario:

- task A is the last user of syscall tracing, it releases the
  TIF_SYSCALL_FTRACE on each tasks

- at the same time task B start syscall tracing. refcount == 0 so
  it sets up TIF_SYSCALL_FTRACE on each tasks.

The effect of the mixup is unpredictable.
So this fix adds a mutex on {start,stop}_syscall_tracing().
Reported-by: NAndrew Morton <akpm@linux-foundation.org>
Reported-by: NIngo Molnar <mingo@elte.hu>
LKML-Reference: <1237151439-6755-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/trace_syscalls.c

@@ -5,7 +5,11 @@
 #include "trace_output.h"
 #include "trace.h"
 
-static atomic_t refcount;
+/* Keep a counter of the syscall tracing users */
+static int refcount;
+
+/* Prevent from races on thread flags toggling */
+static DEFINE_MUTEX(syscall_trace_lock);
 
 /* Option to display the parameters types */
 enum {
@@ -96,9 +100,11 @@ void start_ftrace_syscalls(void)
 	unsigned long flags;
 	struct task_struct *g, *t;
 
+	mutex_lock(&syscall_trace_lock);
+
 	/* Don't enable the flag on the tasks twice */
-	if (atomic_inc_return(&refcount) != 1)
-		return;
+	if (++refcount != 1)
+		goto unlock;
 
 	arch_init_ftrace_syscalls();
 	read_lock_irqsave(&tasklist_lock, flags);
@@ -108,6 +114,9 @@ void start_ftrace_syscalls(void)
 	} while_each_thread(g, t);
 
 	read_unlock_irqrestore(&tasklist_lock, flags);
+
+unlock:
+	mutex_unlock(&syscall_trace_lock);
 }
 
 void stop_ftrace_syscalls(void)
@@ -115,9 +124,11 @@ void stop_ftrace_syscalls(void)
 	unsigned long flags;
 	struct task_struct *g, *t;
 
+	mutex_lock(&syscall_trace_lock);
+
 	/* There are perhaps still some users */
-	if (atomic_dec_return(&refcount))
-		return;
+	if (--refcount)
+		goto unlock;
 
 	read_lock_irqsave(&tasklist_lock, flags);
 
@@ -126,6 +137,9 @@ void stop_ftrace_syscalls(void)
 	} while_each_thread(g, t);
 
 	read_unlock_irqrestore(&tasklist_lock, flags);
+
+unlock:
+	mutex_unlock(&syscall_trace_lock);
 }
 
 void ftrace_syscall_enter(struct pt_regs *regs)