linux-kernel - [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin

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Date:   Fri,  4 May 2018 17:32:18 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, Nicholas Bellinger <nab@...ux-iscsi.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin_lock

percpu_ida() decouples disabling interrupts from the locking operations.
This breaks some assumptions if the locking operations are replaced like
they are under -RT.
The same locking can be achieved by avoiding local_irq_save() and using
spin_lock_irqsave() instead. percpu_ida_alloc() gains one more
preemption point because after unlocking the fastpath and before the
pool lock is acquired, the interrupts are briefly enabled.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 lib/percpu_ida.c | 63 ++++++++++++++++--------------------------------
 1 file changed, 21 insertions(+), 42 deletions(-)

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 6016f1deb1f5..9bbd9c5d375a 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -112,18 +112,6 @@ static inline void alloc_global_tags(struct percpu_ida *pool,
 		  min(pool->nr_free, pool->percpu_batch_size));
 }
 
-static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
-{
-	int tag = -ENOSPC;
-
-	spin_lock(&tags->lock);
-	if (tags->nr_free)
-		tag = tags->freelist[--tags->nr_free];
-	spin_unlock(&tags->lock);
-
-	return tag;
-}
-
 /**
  * percpu_ida_alloc - allocate a tag
  * @pool: pool to allocate from
@@ -147,20 +135,22 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
 	DEFINE_WAIT(wait);
 	struct percpu_ida_cpu *tags;
 	unsigned long flags;
-	int tag;
+	int tag = -ENOSPC;
 
-	local_irq_save(flags);
-	tags = this_cpu_ptr(pool->tag_cpu);
+	tags = raw_cpu_ptr(pool->tag_cpu);
+	spin_lock_irqsave(&tags->lock, flags);
 
 	/* Fastpath */
-	tag = alloc_local_tag(tags);
-	if (likely(tag >= 0)) {
-		local_irq_restore(flags);
+	if (likely(tags->nr_free >= 0)) {
+		tag = tags->freelist[--tags->nr_free];
+		spin_unlock_irqrestore(&tags->lock, flags);
 		return tag;
 	}
+	spin_unlock_irqrestore(&tags->lock, flags);
 
 	while (1) {
-		spin_lock(&pool->lock);
+		spin_lock_irqsave(&pool->lock, flags);
+		tags = this_cpu_ptr(pool->tag_cpu);
 
 		/*
 		 * prepare_to_wait() must come before steal_tags(), in case
@@ -184,8 +174,7 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
 						&pool->cpus_have_tags);
 		}
 
-		spin_unlock(&pool->lock);
-		local_irq_restore(flags);
+		spin_unlock_irqrestore(&pool->lock, flags);
 
 		if (tag >= 0 || state == TASK_RUNNING)
 			break;
@@ -196,9 +185,6 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
 		}
 
 		schedule();
-
-		local_irq_save(flags);
-		tags = this_cpu_ptr(pool->tag_cpu);
 	}
 	if (state != TASK_RUNNING)
 		finish_wait(&pool->wait, &wait);
@@ -222,28 +208,24 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
 
 	BUG_ON(tag >= pool->nr_tags);
 
-	local_irq_save(flags);
-	tags = this_cpu_ptr(pool->tag_cpu);
+	tags = raw_cpu_ptr(pool->tag_cpu);
 
-	spin_lock(&tags->lock);
+	spin_lock_irqsave(&tags->lock, flags);
 	tags->freelist[tags->nr_free++] = tag;
 
 	nr_free = tags->nr_free;
-	spin_unlock(&tags->lock);
 
 	if (nr_free == 1) {
 		cpumask_set_cpu(smp_processor_id(),
 				&pool->cpus_have_tags);
 		wake_up(&pool->wait);
 	}
+	spin_unlock_irqrestore(&tags->lock, flags);
 
 	if (nr_free == pool->percpu_max_size) {
-		spin_lock(&pool->lock);
+		spin_lock_irqsave(&pool->lock, flags);
+		spin_lock(&tags->lock);
 
-		/*
-		 * Global lock held and irqs disabled, don't need percpu
-		 * lock
-		 */
 		if (tags->nr_free == pool->percpu_max_size) {
 			move_tags(pool->freelist, &pool->nr_free,
 				  tags->freelist, &tags->nr_free,
@@ -251,10 +233,9 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
 
 			wake_up(&pool->wait);
 		}
-		spin_unlock(&pool->lock);
+		spin_unlock(&tags->lock);
+		spin_unlock_irqrestore(&pool->lock, flags);
 	}
-
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(percpu_ida_free);
 
@@ -346,29 +327,27 @@ int percpu_ida_for_each_free(struct percpu_ida *pool, percpu_ida_cb fn,
 	struct percpu_ida_cpu *remote;
 	unsigned cpu, i, err = 0;
 
-	local_irq_save(flags);
 	for_each_possible_cpu(cpu) {
 		remote = per_cpu_ptr(pool->tag_cpu, cpu);
-		spin_lock(&remote->lock);
+		spin_lock_irqsave(&remote->lock, flags);
 		for (i = 0; i < remote->nr_free; i++) {
 			err = fn(remote->freelist[i], data);
 			if (err)
 				break;
 		}
-		spin_unlock(&remote->lock);
+		spin_unlock_irqrestore(&remote->lock, flags);
 		if (err)
 			goto out;
 	}
 
-	spin_lock(&pool->lock);
+	spin_lock_irqsave(&pool->lock, flags);
 	for (i = 0; i < pool->nr_free; i++) {
 		err = fn(pool->freelist[i], data);
 		if (err)
 			break;
 	}
-	spin_unlock(&pool->lock);
+	spin_unlock_irqrestore(&pool->lock, flags);
 out:
-	local_irq_restore(flags);
 	return err;
 }
 EXPORT_SYMBOL_GPL(percpu_ida_for_each_free);
-- 
2.17.0