[PATCH] revert "Optimize sys_times for a single thread process"

This patch reverts 'CONFIG_SMP && thread_group_empty()' optimization in
sys_times().  The reason is that the next patch breaks memory ordering which
is needed for that optimization.

tasklist_lock in sys_times() will be eliminated completely by further patch.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 27 insertions, 65 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 3823ec89d7b8..6b2e4cf3e140 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -139,11 +139,7 @@ repeat:
         ptrace_unlink(p);
         BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
         __exit_signal(p);
-        /*
-         * Note that the fastpath in sys_times depends on __exit_signal having
-         * updated the counters before a task is removed from the tasklist of
-         * the process by __unhash_process.
-         */
+
         __unhash_process(p);
 
         /*
diff --git a/kernel/sys.c b/kernel/sys.c
index c93d37f71aef..84371fdc660b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1202,69 +1202,35 @@ asmlinkage long sys_times(struct tms __user * tbuf)
          */
         if (tbuf) {
                 struct tms tmp;
+                struct task_struct *tsk = current;
+                struct task_struct *t;
                 cputime_t utime, stime, cutime, cstime;
 
-#ifdef CONFIG_SMP
-                if (thread_group_empty(current)) {
-                        /*
-                         * Single thread case without the use of any locks.
-                         *
-                         * We may race with release_task if two threads are
-                         * executing. However, release task first adds up the
-                         * counters (__exit_signal) before  removing the task
-                         * from the process tasklist (__unhash_process).
-                         * __exit_signal also acquires and releases the
-                         * siglock which results in the proper memory ordering
-                         * so that the list modifications are always visible
-                         * after the counters have been updated.
-                         *
-                         * If the counters have been updated by the second thread
-                         * but the thread has not yet been removed from the list
-                         * then the other branch will be executing which will
-                         * block on tasklist_lock until the exit handling of the
-                         * other task is finished.
-                         *
-                         * This also implies that the sighand->siglock cannot
-                         * be held by another processor. So we can also
-                         * skip acquiring that lock.
-                         */
-                        utime = cputime_add(current->signal->utime, current->utime);
-                        stime = cputime_add(current->signal->utime, current->stime);
-                        cutime = current->signal->cutime;
-                        cstime = current->signal->cstime;
-                } else
-#endif
-                {
-
-                        /* Process with multiple threads */
-                        struct task_struct *tsk = current;
-                        struct task_struct *t;
-
-                        read_lock(&tasklist_lock);
-                        utime = tsk->signal->utime;
-                        stime = tsk->signal->stime;
-                        t = tsk;
-                        do {
-                                utime = cputime_add(utime, t->utime);
-                                stime = cputime_add(stime, t->stime);
-                                t = next_thread(t);
-                        } while (t != tsk);
+                read_lock(&tasklist_lock);
+                utime = tsk->signal->utime;
+                stime = tsk->signal->stime;
+                t = tsk;
+                do {
+                        utime = cputime_add(utime, t->utime);
+                        stime = cputime_add(stime, t->stime);
+                        t = next_thread(t);
+                } while (t != tsk);
+
+                /*
+                 * While we have tasklist_lock read-locked, no dying thread
+                 * can be updating current->signal->[us]time.  Instead,
+                 * we got their counts included in the live thread loop.
+                 * However, another thread can come in right now and
+                 * do a wait call that updates current->signal->c[us]time.
+                 * To make sure we always see that pair updated atomically,
+                 * we take the siglock around fetching them.
+                 */
+                spin_lock_irq(&tsk->sighand->siglock);
+                cutime = tsk->signal->cutime;
+                cstime = tsk->signal->cstime;
+                spin_unlock_irq(&tsk->sighand->siglock);
+                read_unlock(&tasklist_lock);
 
-                        /*
-                         * While we have tasklist_lock read-locked, no dying thread
-                         * can be updating current->signal->[us]time.  Instead,
-                         * we got their counts included in the live thread loop.
-                         * However, another thread can come in right now and
-                         * do a wait call that updates current->signal->c[us]time.
-                         * To make sure we always see that pair updated atomically,
-                         * we take the siglock around fetching them.
-                         */
-                        spin_lock_irq(&tsk->sighand->siglock);
-                        cutime = tsk->signal->cutime;
-                        cstime = tsk->signal->cstime;
-                        spin_unlock_irq(&tsk->sighand->siglock);
-                        read_unlock(&tasklist_lock);
-                }
                 tmp.tms_utime = cputime_to_clock_t(utime);
                 tmp.tms_stime = cputime_to_clock_t(stime);
                 tmp.tms_cutime = cputime_to_clock_t(cutime);