pwm: sifive: Call pwm_sifive_update_clock() while mutex is held

As was documented in commit 0f02f491 ("pwm: sifive: Reduce time the
controller lock is held") a caller of pwm_sifive_update_clock() must
hold the mutex. So fix pwm_sifive_clock_notifier() to grab the lock.

While this necessity was only documented later, the race exists since
the driver was introduced.

Fixes: 9e37a53e ("pwm: sifive: Add a driver for SiFive SoC PWM")
Reported-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20221018061656.1428111-1-u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-sifive.c

@@ -204,8 +204,11 @@ static int pwm_sifive_clock_notifier(struct notifier_block *nb,
 	struct pwm_sifive_ddata *ddata =
 		container_of(nb, struct pwm_sifive_ddata, notifier);
 
-	if (event == POST_RATE_CHANGE)
+	if (event == POST_RATE_CHANGE) {
+		mutex_lock(&ddata->lock);
 		pwm_sifive_update_clock(ddata, ndata->new_rate);
+		mutex_unlock(&ddata->lock);
+	}
 
 	return NOTIFY_OK;
 }