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Commit 35f5cad8 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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[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: default avatarOleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 6a14c5c9

kernel/exit.c

+1 −5
Original line number Diff line number Diff line
@@ -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);



	/*

kernel/sys.c

+26 −60
Original line number Diff line number Diff line
@@ -1202,43 +1202,9 @@ asmlinkage long sys_times(struct tms __user * tbuf) 
	 */

	if (tbuf) {

		struct tms tmp;

		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;

		cputime_t utime, stime, cutime, cstime;



		read_lock(&tasklist_lock);

		utime = tsk->signal->utime;
@@ -1264,7 +1230,7 @@ asmlinkage long sys_times(struct tms __user * tbuf) 
		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);