kernel/fork.c:copy_process(): consolidate the lockless CLONE_THREAD checks

copy_process() does a lot of "chaotic" initializations and checks
CLONE_THREAD twice before it takes tasklist.  In particular it sets
"p->group_leader = p" and then changes it again under tasklist if
!thread_group_leader(p).

This looks a bit confusing, lets create a single "if (CLONE_THREAD)" block
which initializes ->exit_signal, ->group_leader, and ->tgid.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Sergey Dyasly <dserrg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 17 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 7d6962fb6156..6e6a1c11b3e5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1360,11 +1360,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                         goto bad_fork_cleanup_io;
         }
 
-        p->pid = pid_nr(pid);
-        p->tgid = p->pid;
-        if (clone_flags & CLONE_THREAD)
-                p->tgid = current->tgid;
-
         p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
         /*
          * Clear TID on mm_release()?
@@ -1400,12 +1395,19 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         clear_all_latency_tracing(p);
 
         /* ok, now we should be set up.. */
-        if (clone_flags & CLONE_THREAD)
+        p->pid = pid_nr(pid);
+        if (clone_flags & CLONE_THREAD) {
                 p->exit_signal = -1;
-        else if (clone_flags & CLONE_PARENT)
-                p->exit_signal = current->group_leader->exit_signal;
-        else
-                p->exit_signal = (clone_flags & CSIGNAL);
+                p->group_leader = current->group_leader;
+                p->tgid = current->tgid;
+        } else {
+                if (clone_flags & CLONE_PARENT)
+                        p->exit_signal = current->group_leader->exit_signal;
+                else
+                        p->exit_signal = (clone_flags & CSIGNAL);
+                p->group_leader = p;
+                p->tgid = p->pid;
+        }
 
         p->pdeath_signal = 0;
         p->exit_state = 0;
@@ -1414,15 +1416,13 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
         p->dirty_paused_when = 0;
 
-        /*
-         * Ok, make it visible to the rest of the system.
-         * We dont wake it up yet.
-         */
-        p->group_leader = p;
         INIT_LIST_HEAD(&p->thread_group);
         p->task_works = NULL;
 
-        /* Need tasklist lock for parent etc handling! */
+        /*
+         * Make it visible to the rest of the system, but dont wake it up yet.
+         * Need tasklist lock for parent etc handling!
+         */
         write_lock_irq(&tasklist_lock);
 
         /* CLONE_PARENT re-uses the old parent */
@@ -1476,7 +1476,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                         current->signal->nr_threads++;
                         atomic_inc(&current->signal->live);
                         atomic_inc(&current->signal->sigcnt);
-                        p->group_leader = current->group_leader;
                         list_add_tail_rcu(&p->thread_group,
                                           &p->group_leader->thread_group);
                 }