linux-kernel - Re: [PATCH] kcsan, seqlock: Support seqcount_latch

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Message-ID: <20241031092026.GU33184@noisy.programming.kicks-ass.net>
Date: Thu, 31 Oct 2024 10:20:26 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Rutland <mark.rutland@....com>,
	Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
	linux-kernel@...r.kernel.org,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH] kcsan, seqlock: Support seqcount_latch_t

On Wed, Oct 30, 2024 at 09:48:15PM +0100, Peter Zijlstra wrote:
> diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> index 68d6c1190ac7..4958b40ba6c9 100644
> --- a/kernel/time/sched_clock.c
> +++ b/kernel/time/sched_clock.c
> @@ -102,7 +102,9 @@ unsigned long long notrace sched_clock(void)
>  {
>  	unsigned long long ns;
>  	preempt_disable_notrace();
> +	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
>  	ns = sched_clock_noinstr();
> +	kcsan_atomic_next(0);
>  	preempt_enable_notrace();
>  	return ns;
>  }

You might want to consider also folding something like this in.
That should give this instrumented version instrumentation :-)


diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 68d6c1190ac7..db26f343233f 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -80,7 +80,7 @@ notrace int sched_clock_read_retry(unsigned int seq)
 	return raw_read_seqcount_latch_retry(&cd.seq, seq);
 }
 
-unsigned long long noinstr sched_clock_noinstr(void)
+static __always_inline unsigned long long __sched_clock(void)
 {
 	struct clock_read_data *rd;
 	unsigned int seq;
@@ -98,11 +98,16 @@ unsigned long long noinstr sched_clock_noinstr(void)
 	return res;
 }
 
+unsigned long long noinstr sched_clock_noinstr(void)
+{
+	return __sched_clock();
+}
+
 unsigned long long notrace sched_clock(void)
 {
 	unsigned long long ns;
 	preempt_disable_notrace();
-	ns = sched_clock_noinstr();
+	ns = __sched_clock();
 	preempt_enable_notrace();
 	return ns;
 }