Commit f953ccd0 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

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 Fix the outdated comments about tasklist_lock.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: default avatarOleg 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f6507f83

kernel/exit.c

+11 −12
Original line number Diff line number Diff line
@@ -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);

	}



	/*

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