ia64: salinfo: use a waitqueue instead a sema down/up combo

The only purpose of down_try_lock() followed by up() seems to be to wake
up a possible reader. This patch replaces it with a wake-queue. There is
no locking around cpumask_empty() and the test is re-done in case there
was no hit.
With wait_event_interruptible_lock_irq(,&data_saved_lock) we would probably
be able to get rid of the `retry` label. However we still can return CPU
X which is valid now but later (after the lock dropped) the event may
have been removed because the CPU went offline.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/salinfo.c

@@ -141,7 +141,7 @@ enum salinfo_state {
 
 struct salinfo_data {
 	cpumask_t		cpu_event;	/* which cpus have outstanding events */
-	struct semaphore	mutex;
+	wait_queue_head_t	read_wait;
 	u8			*log_buffer;
 	u64			log_size;
 	u8			*oemdata;	/* decoded oem data */
@@ -182,21 +182,6 @@ struct salinfo_platform_oemdata_parms {
 	int ret;
 };
 
-/* Kick the mutex that tells user space that there is work to do.  Instead of
- * trying to track the state of the mutex across multiple cpus, in user
- * context, interrupt context, non-maskable interrupt context and hotplug cpu,
- * it is far easier just to grab the mutex if it is free then release it.
- *
- * This routine must be called with data_saved_lock held, to make the down/up
- * operation atomic.
- */
-static void
-salinfo_work_to_do(struct salinfo_data *data)
-{
-	(void)(down_trylock(&data->mutex) ?: 0);
-	up(&data->mutex);
-}
-
 static void
 salinfo_platform_oemdata_cpu(void *context)
 {
@@ -258,7 +243,7 @@ salinfo_log_wakeup(int type, u8 *buffer, u64 size, int irqsafe)
 	}
 	cpumask_set_cpu(smp_processor_id(), &data->cpu_event);
 	if (irqsafe) {
-		salinfo_work_to_do(data);
+		wake_up_interruptible(&data->read_wait);
 		spin_unlock_irqrestore(&data_saved_lock, flags);
 	}
 }
@@ -271,14 +256,10 @@ extern void ia64_mlogbuf_dump(void);
 static void
 salinfo_timeout_check(struct salinfo_data *data)
 {
-	unsigned long flags;
 	if (!data->open)
 		return;
-	if (!cpumask_empty(&data->cpu_event)) {
-		spin_lock_irqsave(&data_saved_lock, flags);
-		salinfo_work_to_do(data);
-		spin_unlock_irqrestore(&data_saved_lock, flags);
-	}
+	if (!cpumask_empty(&data->cpu_event))
+		wake_up_interruptible(&data->read_wait);
 }
 
 static void
@@ -308,10 +289,11 @@ salinfo_event_read(struct file *file, char __user *buffer, size_t count, loff_t
 	int i, n, cpu = -1;
 
 retry:
-	if (cpumask_empty(&data->cpu_event) && down_trylock(&data->mutex)) {
+	if (cpumask_empty(&data->cpu_event)) {
 		if (file->f_flags & O_NONBLOCK)
 			return -EAGAIN;
-		if (down_interruptible(&data->mutex))
+		if (wait_event_interruptible(data->read_wait,
+					     !cpumask_empty(&data->cpu_event)))
 			return -EINTR;
 	}
 
@@ -510,7 +492,7 @@ salinfo_log_clear(struct salinfo_data *data, int cpu)
 	if (data->state == STATE_LOG_RECORD) {
 		spin_lock_irqsave(&data_saved_lock, flags);
 		cpumask_set_cpu(cpu, &data->cpu_event);
-		salinfo_work_to_do(data);
+		wake_up_interruptible(&data->read_wait);
 		spin_unlock_irqrestore(&data_saved_lock, flags);
 	}
 	return 0;
@@ -582,7 +564,7 @@ salinfo_cpu_callback(struct notifier_block *nb, unsigned long action, void *hcpu
 		     i < ARRAY_SIZE(salinfo_data);
 		     ++i, ++data) {
 			cpumask_set_cpu(cpu, &data->cpu_event);
-			salinfo_work_to_do(data);
+			wake_up_interruptible(&data->read_wait);
 		}
 		spin_unlock_irqrestore(&data_saved_lock, flags);
 		break;
@@ -640,7 +622,7 @@ salinfo_init(void)
 	for (i = 0; i < ARRAY_SIZE(salinfo_log_name); i++) {
 		data = salinfo_data + i;
 		data->type = i;
-		sema_init(&data->mutex, 1);
+		init_waitqueue_head(&data->read_wait);
 		dir = proc_mkdir(salinfo_log_name[i], salinfo_dir);
 		if (!dir)
 			continue;