linux-kernel - [tip: locking/urgent] seqlock, procfs: Match scoped_seqlock_read() critical section vs. RCU ordering in do_task_stat() to do_io

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Message-ID: <176467099575.498.10639662191686704780.tip-bot2@tip-bot2>
Date: Tue, 02 Dec 2025 10:23:15 -0000
From: "tip-bot2 for Ingo Molnar" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Ingo Molnar <mingo@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Christian Brauner <brauner@...nel.org>,
 Al Viro <viro@...iv.linux.org.uk>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/urgent] seqlock, procfs: Match scoped_seqlock_read()
 critical section vs. RCU ordering in do_task_stat() to do_io_accounting()

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     24bc5ea5c01a7695a1308ac24435810855ec71c9
Gitweb:        https://git.kernel.org/tip/24bc5ea5c01a7695a1308ac24435810855ec71c9
Author:        Ingo Molnar <mingo@...nel.org>
AuthorDate:    Tue, 02 Dec 2025 10:05:10 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 02 Dec 2025 11:21:07 +01:00

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: Ingo Molnar <mingo@...nel.org>
Acked-by: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Link: https://patch.msgid.link/aS6rwnaPbHFCdHp1@gmail.com
---
 fs/proc/array.c | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index cbd4bc4..42932f8 100644
--- 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();
 		}
 	}