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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andrew 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 6016f1d..9bbd9c5 100644 (file)
--- 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);