From f953ccd00615140b5e722ffe2b920da22dfb4db9 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 10 Dec 2014 15:54:48 -0800 Subject: [PATCH] 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 Fix the outdated comments about tasklist_lock.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 Cc: Aaron Tomlin Cc: "Eric W. Biederman" Cc: Rik van Riel Cc: Sterling Alexander Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/exit.c | 23 +++++++++++------------ 1 file 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(¤t->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(¤t->sighand->siglock); } /* -- 2.30.2