blktrace: fix blk_probes_ref chaos

Impact: fix mixed ioctl and ftrace-plugin blktrace use refcount bugs

ioctl-based blktrace allocates bt and registers tracepoints when
ioctl(BLKTRACESETUP), and do all cleanups when ioctl(BLKTRACETEARDOWN).

while ftrace-based blktrace allocates/frees bt when:
  # echo 1/0 > /sys/block/sda/sda1/trace/enable

and registers/unregisters tracepoints when:
  # echo blk/nop > /debugfs/tracing/current_tracer
or
  # echo 1/0 > /debugfs/tracing/tracing_enable

The separatation of allocation and registeration causes 2 problems:

  1. current user-space blktrace still calls ioctl(TEARDOWN) when
     ioctl(SETUP) failed:
       # echo 1 > /sys/block/sda/sda1/trace/enable
       # blktrace /dev/sda
         BLKTRACESETUP: Device or resource busy
         ^C
     and now blk_probes_ref == -1

  2. Another way to make blk_probes_ref == -1:
     # plugin sdb && mount sdb1
     # echo 1 > /sys/block/sdb/sdb1/trace/enable
     # remove sdb

This patch does the allocation and registeration when writing
sdaX/trace/enable.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/blktrace.c

@@ -478,7 +478,7 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 		goto err;
 	}
 
-	if (atomic_add_return(1, &blk_probes_ref) == 1)
+	if (atomic_inc_return(&blk_probes_ref) == 1)
 		blk_register_tracepoints();
 
 	return 0;
@@ -1091,8 +1091,6 @@ static void blk_tracer_print_header(struct seq_file *m)
 
 static void blk_tracer_start(struct trace_array *tr)
 {
-	if (atomic_add_return(1, &blk_probes_ref) == 1)
-		blk_register_tracepoints();
 	trace_flags &= ~TRACE_ITER_CONTEXT_INFO;
 }
 
@@ -1107,15 +1105,10 @@ static int blk_tracer_init(struct trace_array *tr)
 static void blk_tracer_stop(struct trace_array *tr)
 {
 	trace_flags |= TRACE_ITER_CONTEXT_INFO;
-	if (atomic_dec_and_test(&blk_probes_ref))
-		blk_unregister_tracepoints();
 }
 
 static void blk_tracer_reset(struct trace_array *tr)
 {
-	if (!atomic_read(&blk_probes_ref))
-		return;
-
 	blk_tracer_enabled = false;
 	blk_tracer_stop(tr);
 }
@@ -1254,6 +1247,9 @@ static int blk_trace_remove_queue(struct request_queue *q)
 	if (bt == NULL)
 		return -EINVAL;
 
+	if (atomic_dec_and_test(&blk_probes_ref))
+		blk_unregister_tracepoints();
+
 	kfree(bt);
 	return 0;
 }
@@ -1280,6 +1276,9 @@ static int blk_trace_setup_queue(struct request_queue *q, dev_t dev)
 		return -EBUSY;
 	}
 
+	if (atomic_inc_return(&blk_probes_ref) == 1)
+		blk_register_tracepoints();
+
 	return 0;
 }