exit: ptrace: shift "reap dead" code from exit_ptrace() to forget_original_parent()

Now that forget_original_parent() uses ->ptrace_entry for EXIT_DEAD tasks,
we can simply pass "dead_children" list to exit_ptrace() and remove
another release_task() loop.  Plus this way we do not need to drop and
reacquire tasklist_lock.

Also shift the list_empty(ptraced) check, if we want this optimization it
makes sense to eliminate the function call altogether.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
Cc: Sterling Alexander <stalexan@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roland McGrath <roland@hack.frob.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 8 insertions, 27 deletions

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index cc79eff4a1ad..987a73a40ef8 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -52,7 +52,7 @@ extern void ptrace_notify(int exit_code);
 extern void __ptrace_link(struct task_struct *child,
                           struct task_struct *new_parent);
 extern void __ptrace_unlink(struct task_struct *child);
-extern void exit_ptrace(struct task_struct *tracer);
+extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
 #define PTRACE_MODE_READ        0x01
 #define PTRACE_MODE_ATTACH      0x02
 #define PTRACE_MODE_NOAUDIT     0x04
diff --git a/kernel/exit.c b/kernel/exit.c
index 772e9175735c..9c9526d87276 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -553,13 +553,11 @@ static void forget_original_parent(struct task_struct *father)
         LIST_HEAD(dead_children);
 
         write_lock_irq(&tasklist_lock);
-        /*
-         * Note that exit_ptrace() and find_new_reaper() might
-         * drop tasklist_lock and reacquire it.
-         */
-        exit_ptrace(father);
-        reaper = find_new_reaper(father);
+        if (unlikely(!list_empty(&father->ptraced)))
+                exit_ptrace(father, &dead_children);
 
+        /* Can drop and reacquire tasklist_lock */
+        reaper = find_new_reaper(father);
         list_for_each_entry(p, &father->children, sibling) {
                 for_each_thread(p, t) {
                         t->real_parent = reaper;
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 54e75226c2c4..1eb9d90c3af9 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -485,36 +485,19 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 
 /*
  * Detach all tasks we were using ptrace on. Called with tasklist held
- * for writing, and returns with it held too. But note it can release
- * and reacquire the lock.
+ * for writing.
  */
-void exit_ptrace(struct task_struct *tracer)
-        __releases(&tasklist_lock)
-        __acquires(&tasklist_lock)
+void exit_ptrace(struct task_struct *tracer, struct list_head *dead)
 {
         struct task_struct *p, *n;
-        LIST_HEAD(ptrace_dead);
-
-        if (likely(list_empty(&tracer->ptraced)))
-                return;
 
         list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
                 if (unlikely(p->ptrace & PT_EXITKILL))
                         send_sig_info(SIGKILL, SEND_SIG_FORCED, p);
 
                 if (__ptrace_detach(tracer, p))
-                        list_add(&p->ptrace_entry, &ptrace_dead);
-        }
-
-        write_unlock_irq(&tasklist_lock);
-        BUG_ON(!list_empty(&tracer->ptraced));
-
-        list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_entry) {
-                list_del_init(&p->ptrace_entry);
-                release_task(p);
+                        list_add(&p->ptrace_entry, dead);
         }
-
-        write_lock_irq(&tasklist_lock);
 }
 
 int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len)