exit: wait: don't use zombie->real_parent

1. wait_task_zombie() uses p->real_parent to get psig/siglock. This is
   correct but needs tasklist_lock, ->real_parent can exit.

   We can use "current" instead. This is our natural child, its parent
   must be our sub-thread.

2. Read psig/sig outside of ->siglock, ->signal is no longer protected
   by this lock.

3. Fix the outdated comments about tasklist_lock. We can not race with
   __exit_signal(), the whole thread group is dead, nobody but us can
   call it.

   Also clarify the usage of ->stats_lock and ->siglock.

Note: thread_group_cputime_adjusted() is sub-optimal in this case, we
probably want to export cputime_adjust() to avoid thread_group_cputime().
The comment says "all threads" but there are no other threads.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 12 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 20875d6398ae..457673d65934 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1010,8 +1010,8 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
          * Check thread_group_leader() to exclude the traced sub-threads.
          */
         if (state == EXIT_DEAD && thread_group_leader(p)) {
-                struct signal_struct *psig;
-                struct signal_struct *sig;
+                struct signal_struct *sig = p->signal;
+                struct signal_struct *psig = current->signal;
                 unsigned long maxrss;
                 cputime_t tgutime, tgstime;
 
@@ -1023,21 +1023,20 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
                  * accumulate in the parent's signal_struct c* fields.
                  *
                  * We don't bother to take a lock here to protect these
-                 * p->signal fields, because they are only touched by
-                 * __exit_signal, which runs with tasklist_lock
-                 * write-locked anyway, and so is excluded here.  We do
-                 * need to protect the access to parent->signal fields,
-                 * as other threads in the parent group can be right
-                 * here reaping other children at the same time.
+                 * p->signal fields because the whole thread group is dead
+                 * and nobody can change them.
+                 *
+                 * psig->stats_lock also protects us from our sub-theads
+                 * which can reap other children at the same time. Until
+                 * we change k_getrusage()-like users to rely on this lock
+                 * we have to take ->siglock as well.
                  *
                  * We use thread_group_cputime_adjusted() to get times for
                  * the thread group, which consolidates times for all threads
                  * in the group including the group leader.
                  */
                 thread_group_cputime_adjusted(p, &tgutime, &tgstime);
-                spin_lock_irq(&p->real_parent->sighand->siglock);
-                psig = p->real_parent->signal;
-                sig = p->signal;
+                spin_lock_irq(&current->sighand->siglock);
                 write_seqlock(&psig->stats_lock);
                 psig->cutime += tgutime + sig->cutime;
                 psig->cstime += tgstime + sig->cstime;
@@ -1062,7 +1061,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
                 task_io_accounting_add(&psig->ioac, &p->ioac);
                 task_io_accounting_add(&psig->ioac, &sig->ioac);
                 write_sequnlock(&psig->stats_lock);
-                spin_unlock_irq(&p->real_parent->sighand->siglock);
+                spin_unlock_irq(&current->sighand->siglock);
         }
 
         /*