tracing/syscalls: fix missing release of tracing

Impact: fix 'stuck' syscall tracer

The syscall tracer uses a refcounter to enable several users
simultaneously.

But the refcounter did not behave correctly and always restored
its value to 0 after calling start_syscall_tracing(). Therefore,
stop_syscall_tracing() couldn't release correctly the tasks from
tracing.

Also the tracer forgot to reset the buffer when it is released.

Drop the pointless refcount decrement on start_syscall_tracing()
and reset the buffer when we release the tracer.

This fixes two reported issue:

- when we switch from syscall tracer to another tracer, syscall
  tracing continued.

- incorrect use of the refcount.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1237151439-6755-1-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 c72e599230ff58d8a238501a5e734ea29fe65b38..c5fc1d8880f6f5a1e47c60409f581fffcb56dd24 100644 (file)
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -96,8 +96,9 @@ void start_ftrace_syscalls(void)
        unsigned long flags;
        struct task_struct *g, *t;
 
+       /* Don't enable the flag on the tasks twice */
        if (atomic_inc_return(&refcount) != 1)
-               goto out;
+               return;
 
        arch_init_ftrace_syscalls();
        read_lock_irqsave(&tasklist_lock, flags);
@@ -107,8 +108,6 @@ void start_ftrace_syscalls(void)
        } while_each_thread(g, t);
 
        read_unlock_irqrestore(&tasklist_lock, flags);
-out:
-       atomic_dec(&refcount);
 }
 
 void stop_ftrace_syscalls(void)
@@ -116,8 +115,9 @@ void stop_ftrace_syscalls(void)
        unsigned long flags;
        struct task_struct *g, *t;
 
+       /* There are perhaps still some users */
        if (atomic_dec_return(&refcount))
-               goto out;
+               return;
 
        read_lock_irqsave(&tasklist_lock, flags);
 
@@ -126,8 +126,6 @@ void stop_ftrace_syscalls(void)
        } while_each_thread(g, t);
 
        read_unlock_irqrestore(&tasklist_lock, flags);
-out:
-       atomic_inc(&refcount);
 }
 
 void ftrace_syscall_enter(struct pt_regs *regs)
@@ -201,6 +199,7 @@ static int init_syscall_tracer(struct trace_array *tr)
 static void reset_syscall_tracer(struct trace_array *tr)
 {
        stop_ftrace_syscalls();
+       tracing_reset_online_cpus(tr);
 }
 
 static struct trace_event syscall_enter_event = {