[PATCH v2 2/3] tty: Avoid threads files iterations in __do_SAK()

[Kirill Tkhai]

From: Oleg Nesterov <oleg@xxxxxxxxxx>

The patch makes __do_SAK() iterate a next thread files
only in case of the thread's files are different
to previous. I.e., if all threads points the same
files_struct, the files will be iterated only once.

Since all threads have the same files_struct is
the generic case for most Linux systems, this
improvement should clearly speed up __do_SAK()
execution.

Also, for_each_process()/for_each_thread() are
used instead of do_each_thread()/while_each_thread().
This prepares __do_SAK() to become tasklist_lock
free, and will be made in next patch.

[ktkhai: comment written]
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 drivers/tty/tty_io.c |   37 +++++++++++++++++++++++++------------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 84715ba1aee2..89326cee2403 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2704,7 +2704,8 @@ void __do_SAK(struct tty_struct *tty)
 #ifdef TTY_SOFT_SAK
 	tty_hangup(tty);
 #else
-	struct task_struct *g, *p;
+	struct task_struct *p, *t;
+	struct files_struct *files;
 	struct pid *session;
 	int		i;
 
@@ -2725,22 +2726,34 @@ void __do_SAK(struct tty_struct *tty)
 	} while_each_pid_task(session, PIDTYPE_SID, p);
 
 	/* Now kill any processes that happen to have the tty open */
-	do_each_thread(g, p) {
+	for_each_process(p) {
 		if (p->signal->tty == tty) {
 			tty_notice(tty, "SAK: killed process %d (%s): by controlling tty\n",
 				   task_pid_nr(p), p->comm);
-			send_sig(SIGKILL, p, 1);
-			continue;
+			goto kill;
 		}
-		task_lock(p);
-		i = iterate_fd(p->files, 0, this_tty, tty);
-		if (i != 0) {
-			tty_notice(tty, "SAK: killed process %d (%s): by fd#%d\n",
-				   task_pid_nr(p), p->comm, i - 1);
-			send_sig(SIGKILL, p, 1);
+
+		files = NULL;
+		for_each_thread(p, t) {
+			if (t->files == files) /* racy but we do not care */
+				continue;
+
+			task_lock(t);
+			files = t->files;
+			i = iterate_fd(files, 0, this_tty, tty);
+			task_unlock(t);
+
+			if (i != 0) {
+				tty_notice(tty, "SAK: killed process %d (%s): by fd#%d\n",
+					   task_pid_nr(t), t->comm, i - 1);
+				goto kill;
+			}
 		}
-		task_unlock(p);
-	} while_each_thread(g, p);
+
+		continue;
+kill:
+		send_sig(SIGKILL, p, 1);
+	}
 	read_unlock(&tasklist_lock);
 #endif
 }