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Subject[PATCH v2 2/3] tty: Avoid threads files iterations in __do_SAK()
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From: Oleg Nesterov <oleg@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 <oleg@redhat.com>
Reviewed-by: Kirill Tkhai <ktkhai@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
}
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