seqlock, procfs: Match scoped_seqlock_read() critical section vs. RCU ordering in...

There's two patterns of taking the RCU read-lock and the
sig->stats_lock read-seqlock in do_task_stat() and
do_io_accounting(), with a different ordering:

	# do_io_accounting():

	guard(rcu)();
	scoped_seqlock_read (&sig->stats_lock, ss_lock_irqsave) {

	# do_task_stat():

	scoped_seqlock_read (&sig->stats_lock, ss_lock_irqsave) {
	...
			rcu_read_lock();

The ordering is RCU-read+seqlock_read in the first
case, seqlock_read+RCU-read in the second case.

While technically these read locks can be taken in any order,
nevertheless it's good practice to use the more intrusive lock
on the inside (which is the IRQs-off section in this case),
and reduces head-scratching during review when done consistently,
so let's use the do_io_accounting() pattern in do_task_stat().

This will also reduce irqs-off latencies in do_task_stat() a tiny bit.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://patch.msgid.link/aS6rwnaPbHFCdHp1@gmail.com

diff --git a/fs/proc/array.c b/fs/proc/array.c
index cbd4bc4a58e41cddbc0bb9a4760bfd7789f750c2..42932f88141a978b43493a6c720439439cf9e3c0 100644 (file)
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -537,27 +537,27 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
        if (permitted && (!whole || num_threads < 2))
                wchan = !task_is_running(task);
 
-       scoped_seqlock_read (&sig->stats_lock, ss_lock_irqsave) {
-               cmin_flt = sig->cmin_flt;
-               cmaj_flt = sig->cmaj_flt;
-               cutime = sig->cutime;
-               cstime = sig->cstime;
-               cgtime = sig->cgtime;
-
-               if (whole) {
-                       struct task_struct *t;
-
-                       min_flt = sig->min_flt;
-                       maj_flt = sig->maj_flt;
-                       gtime = sig->gtime;
-
-                       rcu_read_lock();
-                       __for_each_thread(sig, t) {
-                               min_flt += t->min_flt;
-                               maj_flt += t->maj_flt;
-                               gtime += task_gtime(t);
+       scoped_guard(rcu) {
+               scoped_seqlock_read (&sig->stats_lock, ss_lock_irqsave) {
+                       cmin_flt = sig->cmin_flt;
+                       cmaj_flt = sig->cmaj_flt;
+                       cutime = sig->cutime;
+                       cstime = sig->cstime;
+                       cgtime = sig->cgtime;
+
+                       if (whole) {
+                               struct task_struct *t;
+
+                               min_flt = sig->min_flt;
+                               maj_flt = sig->maj_flt;
+                               gtime = sig->gtime;
+
+                               __for_each_thread(sig, t) {
+                                       min_flt += t->min_flt;
+                                       maj_flt += t->maj_flt;
+                                       gtime += task_gtime(t);
+                               }
                        }
-                       rcu_read_unlock();
                }
        }