Commit 24bc5ea5 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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


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

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: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarOleg 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
parent 4a26e703

fs/proc/array.c

+20 −20
Original line number Diff line number Diff line
@@ -537,6 +537,7 @@ 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_guard(rcu) {

		scoped_seqlock_read (&sig->stats_lock, ss_lock_irqsave) {

			cmin_flt = sig->cmin_flt;

			cmaj_flt = sig->cmaj_flt;
@@ -551,13 +552,12 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, 
				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);

				}

			rcu_read_unlock();

			}

		}

	}