linux-kernel - [RESEND PATCH v2 09/19] hrtimers: Introduce hrtimer_update

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Message-Id: <20a937b0ae09ad54b5b6d86eabead7c570f1b72e.1730386209.git.namcao@linutronix.de>
Date: Thu, 31 Oct 2024 16:14:23 +0100
From: Nam Cao <namcao@...utronix.de>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Kees Cook <kees@...nel.org>,
	linux-kernel@...r.kernel.org
Cc: Jani Nikula <jani.nikula@...ux.intel.com>,
	intel-gfx@...ts.freedesktop.org,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	x86@...nel.org,
	Jakub Kicinski <kuba@...nel.org>,
	Kalle Valo <kvalo@...nel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Christian Brauner <brauner@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <jstultz@...gle.com>,
	Nam Cao <namcao@...utronix.de>,
	Oliver Hartkopp <socketcan@...tkopp.net>
Subject: [RESEND PATCH v2 09/19] hrtimers: Introduce hrtimer_update_function()

Some users of hrtimer need to change the callback function after the
initial setup. They write to hrtimer::function directly.

That's not safe under all circumstances as the write is lockless and a
concurrent timer expiry might end up using the wrong function pointer.

Introduce hrtimer_update_function(), which also performs runtime checks
whether it is safe to modify the callback.

This allows to make hrtimer::function private once all users are converted.

Signed-off-by: Nam Cao <namcao@...utronix.de>
---
 include/linux/hrtimer.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 48872a2b4071..6e026730e803 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -327,6 +327,28 @@ static inline int hrtimer_callback_running(struct hrtimer *timer)
 	return timer->base->running == timer;
 }
 
+/**
+ * hrtimer_update_function - Update the timer's callback function
+ * @timer:	Timer to update
+ * @function:	New callback function
+ *
+ * Only safe to call if the timer is not enqueued. Can be called in the callback function if the
+ * timer is not enqueued at the same time (see the comments above HRTIMER_STATE_ENQUEUED).
+ */
+static inline void hrtimer_update_function(struct hrtimer *timer,
+					   enum hrtimer_restart (*function)(struct hrtimer *))
+{
+	guard(raw_spinlock_irqsave)(&timer->base->cpu_base->lock);
+
+	if (WARN_ON_ONCE(hrtimer_is_queued(timer)))
+		return;
+
+	if (WARN_ON_ONCE(!function))
+		return;
+
+	timer->function = function;
+}
+
 /* Forward a hrtimer so it expires after now: */
 extern u64
 hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval);
-- 
2.39.5