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Subject[PATCH 08/13] block, cfq: fix race condition in cic creation path and tighten locking
Date
cfq_get_io_context() would fail if multiple tasks race to insert cic's
for the same association. This patch restructures
cfq_get_io_context() such that slow path insertion race is handled
properly.

Note that the restructuring also makes cfq_get_io_context() called
under queue_lock and performs both ioc and cfqd insertions while
holding both ioc and queue locks. This is part of on-going locking
tightening and will be used to simplify synchronization rules.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
---
block/cfq-iosched.c | 135 ++++++++++++++++++++++++++++----------------------
1 files changed, 76 insertions(+), 59 deletions(-)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 51aece2..181a63d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2908,13 +2908,10 @@ static void changed_ioprio(struct cfq_io_context *cic)
{
struct cfq_data *cfqd = cic_to_cfqd(cic);
struct cfq_queue *cfqq;
- unsigned long flags;

if (unlikely(!cfqd))
return;

- spin_lock_irqsave(cfqd->queue->queue_lock, flags);
-
cfqq = cic->cfqq[BLK_RW_ASYNC];
if (cfqq) {
struct cfq_queue *new_cfqq;
@@ -2929,8 +2926,6 @@ static void changed_ioprio(struct cfq_io_context *cic)
cfqq = cic->cfqq[BLK_RW_SYNC];
if (cfqq)
cfq_mark_cfqq_prio_changed(cfqq);
-
- spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
@@ -2958,7 +2953,6 @@ static void changed_cgroup(struct cfq_io_context *cic)
{
struct cfq_queue *sync_cfqq = cic_to_cfqq(cic, 1);
struct cfq_data *cfqd = cic_to_cfqd(cic);
- unsigned long flags;
struct request_queue *q;

if (unlikely(!cfqd))
@@ -2966,8 +2960,6 @@ static void changed_cgroup(struct cfq_io_context *cic)

q = cfqd->queue;

- spin_lock_irqsave(q->queue_lock, flags);
-
if (sync_cfqq) {
/*
* Drop reference to sync queue. A new sync queue will be
@@ -2977,8 +2969,6 @@ static void changed_cgroup(struct cfq_io_context *cic)
cic_set_cfqq(cic, NULL, 1);
cfq_put_queue(sync_cfqq);
}
-
- spin_unlock_irqrestore(q->queue_lock, flags);
}
#endif /* CONFIG_CFQ_GROUP_IOSCHED */

@@ -3142,16 +3132,31 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
return cic;
}

-/*
- * Add cic into ioc, using cfqd as the search key. This enables us to lookup
- * the process specific cfq io context when entered from the block layer.
- * Also adds the cic to a per-cfqd list, used when this queue is removed.
+/**
+ * cfq_create_cic - create and link a cfq_io_context
+ * @cfqd: cfqd of interest
+ * @gfp_mask: allocation mask
+ *
+ * Make sure cfq_io_context linking %current->io_context and @cfqd exists.
+ * If ioc and/or cic doesn't exist, they will be created using @gfp_mask.
*/
-static int cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
- struct cfq_io_context *cic, gfp_t gfp_mask)
+static int cfq_create_cic(struct cfq_data *cfqd, gfp_t gfp_mask)
{
- unsigned long flags;
- int ret;
+ struct request_queue *q = cfqd->queue;
+ struct cfq_io_context *cic = NULL;
+ struct io_context *ioc;
+ int ret = -ENOMEM;
+
+ might_sleep_if(gfp_mask & __GFP_WAIT);
+
+ /* allocate stuff */
+ ioc = current_io_context(gfp_mask, q->node);
+ if (!ioc)
+ goto out;
+
+ cic = cfq_alloc_io_context(cfqd, gfp_mask);
+ if (!cic)
+ goto out;

ret = radix_tree_preload(gfp_mask);
if (ret)
@@ -3161,53 +3166,72 @@ static int cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
cic->key = cfqd;
cic->q = cfqd->queue;

- spin_lock_irqsave(&ioc->lock, flags);
- ret = radix_tree_insert(&ioc->radix_root, cfqd->queue->id, cic);
- if (!ret)
- hlist_add_head_rcu(&cic->cic_list, &ioc->cic_list);
- spin_unlock_irqrestore(&ioc->lock, flags);
-
- radix_tree_preload_end();
+ /* lock both q and ioc and try to link @cic */
+ spin_lock_irq(q->queue_lock);
+ spin_lock(&ioc->lock);

- if (!ret) {
- spin_lock_irqsave(cfqd->queue->queue_lock, flags);
+ ret = radix_tree_insert(&ioc->radix_root, q->id, cic);
+ if (likely(!ret)) {
+ hlist_add_head_rcu(&cic->cic_list, &ioc->cic_list);
list_add(&cic->queue_list, &cfqd->cic_list);
- spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
+ cic = NULL;
+ } else if (ret == -EEXIST) {
+ /* someone else already did it */
+ ret = 0;
}
+
+ spin_unlock(&ioc->lock);
+ spin_unlock_irq(q->queue_lock);
+
+ radix_tree_preload_end();
out:
if (ret)
printk(KERN_ERR "cfq: cic link failed!\n");
+ if (cic)
+ cfq_cic_free(cic);
return ret;
}

-/*
- * Setup general io context and cfq io context. There can be several cfq
- * io contexts per general io context, if this process is doing io to more
- * than one device managed by cfq.
+/**
+ * cfq_get_io_context - acquire cfq_io_context and bump refcnt on io_context
+ * @cfqd: cfqd to setup cic for
+ * @gfp_mask: allocation mask
+ *
+ * Return cfq_io_context associating @cfqd and %current->io_context and
+ * bump refcnt on io_context. If ioc or cic doesn't exist, they're created
+ * using @gfp_mask.
+ *
+ * Must be called under queue_lock which may be released and re-acquired.
+ * This function also may sleep depending on @gfp_mask.
*/
static struct cfq_io_context *
cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
{
- struct io_context *ioc = NULL;
+ struct request_queue *q = cfqd->queue;
struct cfq_io_context *cic = NULL;
+ struct io_context *ioc;
+ int err;
+
+ lockdep_assert_held(q->queue_lock);
+
+ while (true) {
+ /* fast path */
+ ioc = current->io_context;
+ if (likely(ioc)) {
+ cic = cfq_cic_lookup(cfqd, ioc);
+ if (likely(cic))
+ break;
+ }

- might_sleep_if(gfp_mask & __GFP_WAIT);
-
- ioc = current_io_context(gfp_mask, cfqd->queue->node);
- if (!ioc)
- goto err;
-
- cic = cfq_cic_lookup(cfqd, ioc);
- if (cic)
- goto out;
-
- cic = cfq_alloc_io_context(cfqd, gfp_mask);
- if (cic == NULL)
- goto err;
+ /* slow path - unlock, create missing ones and retry */
+ spin_unlock_irq(q->queue_lock);
+ err = cfq_create_cic(cfqd, gfp_mask);
+ spin_lock_irq(q->queue_lock);
+ if (err)
+ return NULL;
+ }

- if (cfq_cic_link(cfqd, ioc, cic, gfp_mask))
- goto err;
-out:
+ /* bump @ioc's refcnt and handle changed notifications */
get_io_context(ioc);

if (unlikely(cic->changed)) {
@@ -3220,10 +3244,6 @@ out:
}

return cic;
-err:
- if (cic)
- cfq_cic_free(cic);
- return NULL;
}

static void
@@ -3759,14 +3779,11 @@ cfq_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
const int rw = rq_data_dir(rq);
const bool is_sync = rq_is_sync(rq);
struct cfq_queue *cfqq;
- unsigned long flags;

might_sleep_if(gfp_mask & __GFP_WAIT);

+ spin_lock_irq(q->queue_lock);
cic = cfq_get_io_context(cfqd, gfp_mask);
-
- spin_lock_irqsave(q->queue_lock, flags);
-
if (!cic)
goto queue_fail;

@@ -3802,12 +3819,12 @@ new_queue:
rq->elevator_private[0] = cic;
rq->elevator_private[1] = cfqq;
rq->elevator_private[2] = cfq_ref_get_cfqg(cfqq->cfqg);
- spin_unlock_irqrestore(q->queue_lock, flags);
+ spin_unlock_irq(q->queue_lock);
return 0;

queue_fail:
cfq_schedule_dispatch(cfqd);
- spin_unlock_irqrestore(q->queue_lock, flags);
+ spin_unlock_irq(q->queue_lock);
cfq_log(cfqd, "set_request fail");
return 1;
}
--
1.7.3.1


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