linux-kernel - [RFC PATCH 2/2] seqlock: introduce need_seqretry

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Date:   Tue, 24 Oct 2023 14:08:40 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Alexey Gladkov <legion@...nel.org>,
        "Ahmed S. Darwish" <darwi@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Waiman Long <longman@...hat.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: [RFC PATCH 2/2] seqlock: introduce need_seqretry_xxx()

Not for inclusion, just for discussion...

Modulo naming, do you think the new need_seqretry_xxx() makes sense?

Simpler to use and less error prone. thread_group_cputime() is changed
as an example.
---
 include/linux/seqlock.h | 10 ++++++++++
 kernel/sched/cputime.c  |  9 +++------
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d0050c889f26..9b3bc4ce3332 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1165,6 +1165,16 @@ static inline int need_seqretry(seqlock_t *lock, int seq)
 	return !(seq & 1) && read_seqretry(lock, seq);
 }
 
+static inline int need_seqretry_xxx(seqlock_t *lock, int *seq)
+{
+	int ret = !(*seq & 1) && read_seqretry(lock, *seq);
+
+	if (ret)
+		++*seq;	/* make this counter odd */
+
+	return ret;
+}
+
 /**
  * done_seqretry() - end seqlock_t "locking or lockless" reader section
  * @lock: Pointer to seqlock_t
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index af7952f12e6c..45704a84baec 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -314,7 +314,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 	struct signal_struct *sig = tsk->signal;
 	u64 utime, stime;
 	struct task_struct *t;
-	unsigned int seq, nextseq;
+	unsigned int seq;
 	unsigned long flags;
 
 	/*
@@ -330,9 +330,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 
 	rcu_read_lock();
 	/* Attempt a lockless read on the first round. */
-	nextseq = 0;
+	seq = 0;
 	do {
-		seq = nextseq;
 		flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
 		times->utime = sig->utime;
 		times->stime = sig->stime;
@@ -344,9 +343,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
 			times->stime += stime;
 			times->sum_exec_runtime += read_sum_exec_runtime(t);
 		}
-		/* If lockless access failed, take the lock. */
-		nextseq = 1;
-	} while (need_seqretry(&sig->stats_lock, seq));
+	} while (need_seqretry_xxx(&sig->stats_lock, &seq));
 	done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
 	rcu_read_unlock();
 }
-- 
2.25.1.362.g51ebf55