nilfs2: remove sc_timer_task

After commit f5d4e04 ("nilfs2: fix use-after-free of timer for log
writer thread") is applied, nilfs_construct_timeout(), which is called by
a timer and wakes up the log writer thread, is never called after the log
writer thread has terminated.

As a result, the member variable "sc_timer_task" of the "nilfs_sc_info"
structure, which was added when timer_setup() was adopted to retain a
reference to the log writer thread's task even after it had terminated, is
no longer needed, as it should be; we can simply use "sc_task" instead,
which holds a reference to the log writer thread's task for its lifetime. 
So, eliminate "sc_timer_task" by this means.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ryusuke Konishi <[email protected]>
Cc: Huang Xiaojia <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: konis

fs/nilfs2/segment.c

@@ -2502,7 +2502,7 @@ static void nilfs_construction_timeout(struct timer_list *t)
 {
 	struct nilfs_sc_info *sci = from_timer(sci, t, sc_timer);
 
-	wake_up_process(sci->sc_timer_task);
+	wake_up_process(sci->sc_task);
 }
 
 static void
@@ -2640,7 +2640,6 @@ static int nilfs_segctor_thread(void *arg)
 	struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
 	int timeout = 0;
 
-	sci->sc_timer_task = current;
 	timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);
 
 	/* start sync. */

fs/nilfs2/segment.h

@@ -115,7 +115,6 @@ struct nilfs_segsum_pointer {
  * @sc_lseg_stime: Start time of the latest logical segment
  * @sc_watermark: Watermark for the number of dirty buffers
  * @sc_timer: Timer for segctord
- * @sc_timer_task: Thread woken up by @sc_timer
  * @sc_task: current thread of segctord
  */
 struct nilfs_sc_info {
@@ -172,7 +171,6 @@ struct nilfs_sc_info {
 	unsigned long		sc_watermark;
 
 	struct timer_list	sc_timer;
-	struct task_struct     *sc_timer_task;
 	struct task_struct     *sc_task;
 };