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Commit 804209d8 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Linus Torvalds
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lib/percpu_ida.c: 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.

Link: http://lkml.kernel.org/r/20180504153218.7301-1-bigeasy@linutronix.de


Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Shaohua Li <shli@fb.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b94078e6

lib/percpu_ida.c

+21 −42
Original line number Diff line number Diff line
@@ -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);