workqueue: split apply_workqueue_attrs() into 3 stages

Current apply_workqueue_attrs() includes pwqs-allocation and pwqs-installation,
so when we batch multiple apply_workqueue_attrs()s as a transaction, we can't
ensure the transaction must succeed or fail as a complete unit.

To solve this, we split apply_workqueue_attrs() into three stages.
The first stage does the preparation: allocation memory, pwqs.
The second stage does the attrs-installaion and pwqs-installation.
The third stage frees the allocated memory and (old or unused) pwqs.

As the result, batching multiple apply_workqueue_attrs()s can
succeed or fail as a complete unit:
	1) batch do all the first stage for all the workqueues
	2) only commit all when all the above succeed.

This patch is a preparation for the next patch ("Allow modifying low level
unbound workqueue cpumask") which will do a multiple apply_workqueue_attrs().

The patch doesn't have functionality changed except two minor adjustment:
	1) free_unbound_pwq() for the error path is removed, we use the
	   heavier version put_pwq_unlocked() instead since the error path
	   is rare. this adjustment simplifies the code.
	2) the memory-allocation is also moved into wq_pool_mutex.
	   this is needed to avoid to do the further splitting.

tj: minor updates to comments.

Suggested-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Mike Galbraith <bitbucket@online.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3425,17 +3425,6 @@ static struct pool_workqueue *alloc_unbound_pwq(struct workqueue_struct *wq,
 	return pwq;
 }
 
-/* undo alloc_unbound_pwq(), used only in the error path */
-static void free_unbound_pwq(struct pool_workqueue *pwq)
-{
-	lockdep_assert_held(&wq_pool_mutex);
-
-	if (pwq) {
-		put_unbound_pool(pwq->pool);
-		kmem_cache_free(pwq_cache, pwq);
-	}
-}
-
 /**
  * wq_calc_node_mask - calculate a wq_attrs' cpumask for the specified node
  * @attrs: the wq_attrs of interest
@@ -3498,42 +3487,48 @@ static struct pool_workqueue *numa_pwq_tbl_install(struct workqueue_struct *wq,
 	return old_pwq;
 }
 
-/**
- * apply_workqueue_attrs - apply new workqueue_attrs to an unbound workqueue
- * @wq: the target workqueue
- * @attrs: the workqueue_attrs to apply, allocated with alloc_workqueue_attrs()
- *
- * Apply @attrs to an unbound workqueue @wq.  Unless disabled, on NUMA
- * machines, this function maps a separate pwq to each NUMA node with
- * possibles CPUs in @attrs->cpumask so that work items are affine to the
- * NUMA node it was issued on.  Older pwqs are released as in-flight work
- * items finish.  Note that a work item which repeatedly requeues itself
- * back-to-back will stay on its current pwq.
- *
- * Performs GFP_KERNEL allocations.
- *
- * Return: 0 on success and -errno on failure.
- */
-int apply_workqueue_attrs(struct workqueue_struct *wq,
-			  const struct workqueue_attrs *attrs)
+/* context to store the prepared attrs & pwqs before applying */
+struct apply_wqattrs_ctx {
+	struct workqueue_struct	*wq;		/* target workqueue */
+	struct workqueue_attrs	*attrs;		/* attrs to apply */
+	struct pool_workqueue	*dfl_pwq;
+	struct pool_workqueue	*pwq_tbl[];
+};
+
+/* free the resources after success or abort */
+static void apply_wqattrs_cleanup(struct apply_wqattrs_ctx *ctx)
+{
+	if (ctx) {
+		int node;
+
+		for_each_node(node)
+			put_pwq_unlocked(ctx->pwq_tbl[node]);
+		put_pwq_unlocked(ctx->dfl_pwq);
+
+		free_workqueue_attrs(ctx->attrs);
+
+		kfree(ctx);
+	}
+}
+
+/* allocate the attrs and pwqs for later installation */
+static struct apply_wqattrs_ctx *
+apply_wqattrs_prepare(struct workqueue_struct *wq,
+		      const struct workqueue_attrs *attrs)
 {
+	struct apply_wqattrs_ctx *ctx;
 	struct workqueue_attrs *new_attrs, *tmp_attrs;
-	struct pool_workqueue **pwq_tbl, *dfl_pwq;
-	int node, ret;
+	int node;
 
-	/* only unbound workqueues can change attributes */
-	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
-		return -EINVAL;
+	lockdep_assert_held(&wq_pool_mutex);
 
-	/* creating multiple pwqs breaks ordering guarantee */
-	if (WARN_ON((wq->flags & __WQ_ORDERED) && !list_empty(&wq->pwqs)))
-		return -EINVAL;
+	ctx = kzalloc(sizeof(*ctx) + nr_node_ids * sizeof(ctx->pwq_tbl[0]),
+		      GFP_KERNEL);
 
-	pwq_tbl = kzalloc(nr_node_ids * sizeof(pwq_tbl[0]), GFP_KERNEL);
 	new_attrs = alloc_workqueue_attrs(GFP_KERNEL);
 	tmp_attrs = alloc_workqueue_attrs(GFP_KERNEL);
-	if (!pwq_tbl || !new_attrs || !tmp_attrs)
-		goto enomem;
+	if (!ctx || !new_attrs || !tmp_attrs)
+		goto out_free;
 
 	/* make a copy of @attrs and sanitize it */
 	copy_workqueue_attrs(new_attrs, attrs);
@@ -3546,76 +3541,112 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 	 */
 	copy_workqueue_attrs(tmp_attrs, new_attrs);
 
-	/*
-	 * CPUs should stay stable across pwq creations and installations.
-	 * Pin CPUs, determine the target cpumask for each node and create
-	 * pwqs accordingly.
-	 */
-	get_online_cpus();
-
-	mutex_lock(&wq_pool_mutex);
-
 	/*
 	 * If something goes wrong during CPU up/down, we'll fall back to
 	 * the default pwq covering whole @attrs->cpumask.  Always create
 	 * it even if we don't use it immediately.
 	 */
-	dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
-	if (!dfl_pwq)
-		goto enomem_pwq;
+	ctx->dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
+	if (!ctx->dfl_pwq)
+		goto out_free;
 
 	for_each_node(node) {
 		if (wq_calc_node_cpumask(attrs, node, -1, tmp_attrs->cpumask)) {
-			pwq_tbl[node] = alloc_unbound_pwq(wq, tmp_attrs);
-			if (!pwq_tbl[node])
-				goto enomem_pwq;
+			ctx->pwq_tbl[node] = alloc_unbound_pwq(wq, tmp_attrs);
+			if (!ctx->pwq_tbl[node])
+				goto out_free;
 		} else {
-			dfl_pwq->refcnt++;
-			pwq_tbl[node] = dfl_pwq;
+			ctx->dfl_pwq->refcnt++;
+			ctx->pwq_tbl[node] = ctx->dfl_pwq;
 		}
 	}
 
-	mutex_unlock(&wq_pool_mutex);
+	ctx->attrs = new_attrs;
+	ctx->wq = wq;
+	free_workqueue_attrs(tmp_attrs);
+	return ctx;
+
+out_free:
+	free_workqueue_attrs(tmp_attrs);
+	free_workqueue_attrs(new_attrs);
+	apply_wqattrs_cleanup(ctx);
+	return NULL;
+}
+
+/* set attrs and install prepared pwqs, @ctx points to old pwqs on return */
+static void apply_wqattrs_commit(struct apply_wqattrs_ctx *ctx)
+{
+	int node;
 
 	/* all pwqs have been created successfully, let's install'em */
-	mutex_lock(&wq->mutex);
+	mutex_lock(&ctx->wq->mutex);
 
-	copy_workqueue_attrs(wq->unbound_attrs, new_attrs);
+	copy_workqueue_attrs(ctx->wq->unbound_attrs, ctx->attrs);
 
 	/* save the previous pwq and install the new one */
 	for_each_node(node)
-		pwq_tbl[node] = numa_pwq_tbl_install(wq, node, pwq_tbl[node]);
+		ctx->pwq_tbl[node] = numa_pwq_tbl_install(ctx->wq, node,
+							  ctx->pwq_tbl[node]);
 
 	/* @dfl_pwq might not have been used, ensure it's linked */
-	link_pwq(dfl_pwq);
-	swap(wq->dfl_pwq, dfl_pwq);
+	link_pwq(ctx->dfl_pwq);
+	swap(ctx->wq->dfl_pwq, ctx->dfl_pwq);
 
-	mutex_unlock(&wq->mutex);
+	mutex_unlock(&ctx->wq->mutex);
+}
 
-	/* put the old pwqs */
-	for_each_node(node)
-		put_pwq_unlocked(pwq_tbl[node]);
-	put_pwq_unlocked(dfl_pwq);
+/**
+ * apply_workqueue_attrs - apply new workqueue_attrs to an unbound workqueue
+ * @wq: the target workqueue
+ * @attrs: the workqueue_attrs to apply, allocated with alloc_workqueue_attrs()
+ *
+ * Apply @attrs to an unbound workqueue @wq.  Unless disabled, on NUMA
+ * machines, this function maps a separate pwq to each NUMA node with
+ * possibles CPUs in @attrs->cpumask so that work items are affine to the
+ * NUMA node it was issued on.  Older pwqs are released as in-flight work
+ * items finish.  Note that a work item which repeatedly requeues itself
+ * back-to-back will stay on its current pwq.
+ *
+ * Performs GFP_KERNEL allocations.
+ *
+ * Return: 0 on success and -errno on failure.
+ */
+int apply_workqueue_attrs(struct workqueue_struct *wq,
+			  const struct workqueue_attrs *attrs)
+{
+	struct apply_wqattrs_ctx *ctx;
+	int ret = -ENOMEM;
 
-	put_online_cpus();
-	ret = 0;
-	/* fall through */
-out_free:
-	free_workqueue_attrs(tmp_attrs);
-	free_workqueue_attrs(new_attrs);
-	kfree(pwq_tbl);
-	return ret;
+	/* only unbound workqueues can change attributes */
+	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
+		return -EINVAL;
 
-enomem_pwq:
-	free_unbound_pwq(dfl_pwq);
-	for_each_node(node)
-		if (pwq_tbl && pwq_tbl[node] != dfl_pwq)
-			free_unbound_pwq(pwq_tbl[node]);
+	/* creating multiple pwqs breaks ordering guarantee */
+	if (WARN_ON((wq->flags & __WQ_ORDERED) && !list_empty(&wq->pwqs)))
+		return -EINVAL;
+
+	/*
+	 * CPUs should stay stable across pwq creations and installations.
+	 * Pin CPUs, determine the target cpumask for each node and create
+	 * pwqs accordingly.
+	 */
+	get_online_cpus();
+
+	mutex_lock(&wq_pool_mutex);
+	ctx = apply_wqattrs_prepare(wq, attrs);
 	mutex_unlock(&wq_pool_mutex);
+
+	/* the ctx has been prepared successfully, let's commit it */
+	if (ctx) {
+		apply_wqattrs_commit(ctx);
+		ret = 0;
+	}
+
 	put_online_cpus();
-enomem:
-	ret = -ENOMEM;
-	goto out_free;
+
+	apply_wqattrs_cleanup(ctx);
+
+	return ret;
 }
 
 /**