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Subject[patch 64/87] ptrace: simplify ptrace_exit()->ignoring_children() path
2.6.29-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Oleg Nesterov <oleg@redhat.com>

commit 6d69cb87f05eef3b02370b2f7bae608ad2301a00 upstream.

ignoring_children() takes parent->sighand->siglock and checks
k_sigaction[SIGCHLD] atomically. But this buys nothing, we can't get the
"really" wrong result even if we race with sigaction(SIGCHLD). If we read
the "stale" sa_handler/sa_flags we can pretend it was changed right after
the check.

Remove spin_lock(->siglock), and kill "int ign" which caches the result of
ignoring_children() which becomes rather trivial.

Perhaps it makes sense to export this helper, do_notify_parent() can use
it too.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/exit.c | 25 ++++++++-----------------
1 file changed, 8 insertions(+), 17 deletions(-)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -703,19 +703,15 @@ static void exit_mm(struct task_struct *
}

/*
- * Return nonzero if @parent's children should reap themselves.
- *
- * Called with write_lock_irq(&tasklist_lock) held.
+ * Called with irqs disabled, returns true if childs should reap themselves.
*/
-static int ignoring_children(struct task_struct *parent)
+static int ignoring_children(struct sighand_struct *sigh)
{
int ret;
- struct sighand_struct *psig = parent->sighand;
- unsigned long flags;
- spin_lock_irqsave(&psig->siglock, flags);
- ret = (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
- (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT));
- spin_unlock_irqrestore(&psig->siglock, flags);
+ spin_lock(&sigh->siglock);
+ ret = (sigh->action[SIGCHLD-1].sa.sa_handler == SIG_IGN) ||
+ (sigh->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT);
+ spin_unlock(&sigh->siglock);
return ret;
}

@@ -728,7 +724,6 @@ static int ignoring_children(struct task
static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
{
struct task_struct *p, *n;
- int ign = -1;

list_for_each_entry_safe(p, n, &parent->ptraced, ptrace_entry) {
__ptrace_unlink(p);
@@ -750,12 +745,8 @@ static void ptrace_exit(struct task_stru
if (!task_detached(p) && thread_group_empty(p)) {
if (!same_thread_group(p->real_parent, parent))
do_notify_parent(p, p->exit_signal);
- else {
- if (ign < 0)
- ign = ignoring_children(parent);
- if (ign)
- p->exit_signal = -1;
- }
+ else if (ignoring_children(parent->sighand))
+ p->exit_signal = -1;
}

if (task_detached(p)) {



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