4 files changed, 81 insertions, 46 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 8f630ce..4b001dc 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1645,11 +1645,12 @@ static void blkiocg_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
 	struct io_context *ioc;
 
-	task_lock(tsk);
-	ioc = tsk->io_context;
-	if (ioc)
+	/* we don't lose anything even if ioc allocation fails */
+	ioc = get_task_io_context(tsk, GFP_ATOMIC, NUMA_NO_NODE);
+	if (ioc) {
 		ioc->cgroup_changed = 1;
-	task_unlock(tsk);
+		put_io_context(ioc);
+	}
 }
 
 void blkio_policy_register(struct blkio_policy_type *blkiop)
diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index 8bebf06..b13ed96 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -16,6 +16,19 @@
  */
 static struct kmem_cache *iocontext_cachep;
 
+/**
+ * get_io_context - increment reference count to io_context
+ * @ioc: io_context to get
+ *
+ * Increment reference count to @ioc.
+ */
+void get_io_context(struct io_context *ioc)
+{
+	BUG_ON(atomic_long_read(&ioc->refcount) <= 0);
+	atomic_long_inc(&ioc->refcount);
+}
+EXPORT_SYMBOL(get_io_context);
+
 static void cfq_dtor(struct io_context *ioc)
 {
 	if (!hlist_empty(&ioc->cic_list)) {
@@ -71,6 +84,9 @@ void exit_io_context(struct task_struct *task)
 {
 	struct io_context *ioc;
 
+	/* PF_EXITING prevents new io_context from being attached to @task */
+	WARN_ON_ONCE(!(current->flags & PF_EXITING));
+
 	task_lock(task);
 	ioc = task->io_context;
 	task->io_context = NULL;
@@ -82,7 +98,9 @@ void exit_io_context(struct task_struct *task)
 	put_io_context(ioc);
 }
 
-struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
+static struct io_context *create_task_io_context(struct task_struct *task,
+						 gfp_t gfp_flags, int node,
+						 bool take_ref)
 {
 	struct io_context *ioc;
 
@@ -98,6 +116,20 @@ struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
 	INIT_RADIX_TREE(&ioc->radix_root, GFP_ATOMIC | __GFP_HIGH);
 	INIT_HLIST_HEAD(&ioc->cic_list);
 
+	/* try to install, somebody might already have beaten us to it */
+	task_lock(task);
+
+	if (!task->io_context && !(task->flags & PF_EXITING)) {
+		task->io_context = ioc;
+	} else {
+		kmem_cache_free(iocontext_cachep, ioc);
+		ioc = task->io_context;
+	}
+
+	if (ioc && take_ref)
+		get_io_context(ioc);
+
+	task_unlock(task);
 	return ioc;
 }
 
@@ -114,46 +146,47 @@ struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
  */
 struct io_context *current_io_context(gfp_t gfp_flags, int node)
 {
-	struct task_struct *tsk = current;
-	struct io_context *ret;
-
-	ret = tsk->io_context;
-	if (likely(ret))
-		return ret;
-
-	ret = alloc_io_context(gfp_flags, node);
-	if (ret) {
-		/* make sure set_task_ioprio() sees the settings above */
-		smp_wmb();
-		tsk->io_context = ret;
-	}
+	might_sleep_if(gfp_flags & __GFP_WAIT);
 
-	return ret;
+	if (current->io_context)
+		return current->io_context;
+
+	return create_task_io_context(current, gfp_flags, node, false);
 }
+EXPORT_SYMBOL(current_io_context);
 
-/*
- * If the current task has no IO context then create one and initialise it.
- * If it does have a context, take a ref on it.
+/**
+ * get_task_io_context - get io_context of a task
+ * @task: task of interest
+ * @gfp_flags: allocation flags, used if allocation is necessary
+ * @node: allocation node, used if allocation is necessary
+ *
+ * Return io_context of @task.  If it doesn't exist, it is created with
+ * @gfp_flags and @node.  The returned io_context has its reference count
+ * incremented.
  *
- * This is always called in the context of the task which submitted the I/O.
+ * This function always goes through task_lock() and it's better to use
+ * current_io_context() + get_io_context() for %current.
  */
-struct io_context *get_io_context(gfp_t gfp_flags, int node)
+struct io_context *get_task_io_context(struct task_struct *task,
+				       gfp_t gfp_flags, int node)
 {
-	struct io_context *ioc = NULL;
-
-	/*
-	 * Check for unlikely race with exiting task. ioc ref count is
-	 * zero when ioc is being detached.
-	 */
-	do {
-		ioc = current_io_context(gfp_flags, node);
-		if (unlikely(!ioc))
-			break;
-	} while (!atomic_long_inc_not_zero(&ioc->refcount));
+	struct io_context *ioc;
 
-	return ioc;
+	might_sleep_if(gfp_flags & __GFP_WAIT);
+
+	task_lock(task);
+	ioc = task->io_context;
+	if (likely(ioc)) {
+		get_io_context(ioc);
+		task_unlock(task);
+		return ioc;
+	}
+	task_unlock(task);
+
+	return create_task_io_context(task, gfp_flags, node, true);
 }
-EXPORT_SYMBOL(get_io_context);
+EXPORT_SYMBOL(get_task_io_context);
 
 static int __init blk_ioc_init(void)
 {
diff --git a/block/blk.h b/block/blk.h
index aae4d88..fc3c41b 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -122,6 +122,7 @@ static inline int blk_should_fake_timeout(struct request_queue *q)
 }
 #endif
 
+void get_io_context(struct io_context *ioc);
 struct io_context *current_io_context(gfp_t gfp_flags, int node);
 
 int ll_back_merge_fn(struct request_queue *q, struct request *req,
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ec3f5e8b..d42d89c 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -14,6 +14,7 @@
 #include <linux/rbtree.h>
 #include <linux/ioprio.h>
 #include <linux/blktrace_api.h>
+#include "blk.h"
 #include "cfq.h"
 
 /*
@@ -3194,13 +3195,13 @@ static struct cfq_io_context *
 cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
 {
 	struct io_context *ioc = NULL;
-	struct cfq_io_context *cic;
+	struct cfq_io_context *cic = NULL;
 
 	might_sleep_if(gfp_mask & __GFP_WAIT);
 
-	ioc = get_io_context(gfp_mask, cfqd->queue->node);
+	ioc = current_io_context(gfp_mask, cfqd->queue->node);
 	if (!ioc)
-		return NULL;
+		goto err;
 
 	cic = cfq_cic_lookup(cfqd, ioc);
 	if (cic)
@@ -3211,10 +3212,10 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
 		goto err;
 
 	if (cfq_cic_link(cfqd, ioc, cic, gfp_mask))
-		goto err_free;
-
+		goto err;
 out:
-	smp_read_barrier_depends();
+	get_io_context(ioc);
+
 	if (unlikely(ioc->ioprio_changed))
 		cfq_ioc_set_ioprio(ioc);
 
@@ -3223,10 +3224,9 @@ out:
 		cfq_ioc_set_cgroup(ioc);
 #endif
 	return cic;
-err_free:
-	cfq_cic_free(cic);
 err:
-	put_io_context(ioc);
+	if (cic)
+		cfq_cic_free(cic);
 	return NULL;
 }