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: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1237151439-6755-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 26f9a8679d3db463b7fbb6ce816136bad479adbc..a2a3af29c94337bef68f64608a85977a2fe39ce8 100644 (file)
--- a/kernel/trace/trace_syscalls.c
+++ b/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)