posix-cpu-timers: avoid "task->signal != NULL" checks

Preparation to make task->signal immutable, no functional changes.

posix-cpu-timers.c checks task->signal != NULL to ensure this task is
alive and didn't pass __exit_signal().  This is correct but we are going
to change the lifetime rules for ->signal and never reset this pointer.

Change the code to check ->sighand instead, it doesn't matter which
pointer we check under tasklist, they both are cleared simultaneously.

As Roland pointed out, some of these changes are not strictly needed and
probably it makes sense to revert them later, when ->signal will be pinned
to task_struct.  But this patch tries to ensure the subsequent changes in
fork/exit can't make any visible impact on posix cpu timers.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/posix-cpu-timers.c

@@ -363,7 +363,7 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 				}
 			} else {
 				read_lock(&tasklist_lock);
-				if (thread_group_leader(p) && p->signal) {
+				if (thread_group_leader(p) && p->sighand) {
 					error =
 					    cpu_clock_sample_group(which_clock,
 							           p, &rtn);
@@ -439,7 +439,7 @@ int posix_cpu_timer_del(struct k_itimer *timer)
 
 	if (likely(p != NULL)) {
 		read_lock(&tasklist_lock);
-		if (unlikely(p->signal == NULL)) {
+		if (unlikely(p->sighand == NULL)) {
 			/*
 			 * We raced with the reaping of the task.
 			 * The deletion should have cleared us off the list.
@@ -691,10 +691,10 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 	read_lock(&tasklist_lock);
 	/*
 	 * We need the tasklist_lock to protect against reaping that
-	 * clears p->signal.  If p has just been reaped, we can no
+	 * clears p->sighand.  If p has just been reaped, we can no
 	 * longer get any information about it at all.
 	 */
-	if (unlikely(p->signal == NULL)) {
+	if (unlikely(p->sighand == NULL)) {
 		read_unlock(&tasklist_lock);
 		put_task_struct(p);
 		timer->it.cpu.task = NULL;
@@ -863,7 +863,7 @@ void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 		clear_dead = p->exit_state;
 	} else {
 		read_lock(&tasklist_lock);
-		if (unlikely(p->signal == NULL)) {
+		if (unlikely(p->sighand == NULL)) {
 			/*
 			 * The process has been reaped.
 			 * We can't even collect a sample any more.
@@ -1199,7 +1199,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 		spin_lock(&p->sighand->siglock);
 	} else {
 		read_lock(&tasklist_lock);
-		if (unlikely(p->signal == NULL)) {
+		if (unlikely(p->sighand == NULL)) {
 			/*
 			 * The process has been reaped.
 			 * We can't even collect a sample any more.