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

[Kirill Tkhai]

From: Oleg Nesterov <o...@redhat.com>

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 <o...@redhat.com>
Reviewed-by: Kirill Tkhai <ktk...@virtuozzo.com>
---
 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
 }