There are cases where RCU callback needs to be bounced to a sleepable
context.  This is currently done by the RCU callback queueing a work
item, which can be cumbersome to write and confusing to read.

This patch introduces rcu_work, a workqueue work variant which gets
executed after a RCU grace period, and converts the open coded
bouncing in fs/aio and kernel/cgroup.

v2: Use rcu_barrier() instead of synchronize_rcu() to wait for
    completion of previously queued rcu callback as per Paul.

Signed-off-by: Tejun Heo <t...@kernel.org>
Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
---
 fs/aio.c                    | 21 +++++-------------
 include/linux/cgroup-defs.h |  2 +-
 include/linux/workqueue.h   | 38 +++++++++++++++++++++++++++++++
 kernel/cgroup/cgroup.c      | 21 ++++++------------
 kernel/workqueue.c          | 54 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 106 insertions(+), 30 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 6bcd3fb..88d7927 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -115,8 +115,7 @@ struct kioctx {
        struct page             **ring_pages;
        long                    nr_pages;
 
-       struct rcu_head         free_rcu;
-       struct work_struct      free_work;      /* see free_ioctx() */
+       struct rcu_work         free_rwork;     /* see free_ioctx() */
 
        /*
         * signals when all in-flight requests are done
@@ -592,13 +591,12 @@ static int kiocb_cancel(struct aio_kiocb *kiocb)
 /*
  * free_ioctx() should be RCU delayed to synchronize against the RCU
  * protected lookup_ioctx() and also needs process context to call
- * aio_free_ring(), so the double bouncing through kioctx->free_rcu and
- * ->free_work.
+ * aio_free_ring().  Use rcu_work.
  */
 static void free_ioctx(struct work_struct *work)
 {
-       struct kioctx *ctx = container_of(work, struct kioctx, free_work);
-
+       struct kioctx *ctx = container_of(to_rcu_work(work), struct kioctx,
+                                         free_rwork);
        pr_debug("freeing %p\n", ctx);
 
        aio_free_ring(ctx);
@@ -608,14 +606,6 @@ static void free_ioctx(struct work_struct *work)
        kmem_cache_free(kioctx_cachep, ctx);
 }
 
-static void free_ioctx_rcufn(struct rcu_head *head)
-{
-       struct kioctx *ctx = container_of(head, struct kioctx, free_rcu);
-
-       INIT_WORK(&ctx->free_work, free_ioctx);
-       schedule_work(&ctx->free_work);
-}
-
 static void free_ioctx_reqs(struct percpu_ref *ref)
 {
        struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
@@ -625,7 +615,8 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
                complete(&ctx->rq_wait->comp);
 
        /* Synchronize against RCU protected table->table[] dereferences */
-       call_rcu(&ctx->free_rcu, free_ioctx_rcufn);
+       INIT_RCU_WORK(&ctx->free_rwork, free_ioctx);
+       queue_rcu_work(system_wq, &ctx->free_rwork);
 }
 
 /*
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 9f242b8..92d7640 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -151,8 +151,8 @@ struct cgroup_subsys_state {
        atomic_t online_cnt;
 
        /* percpu_ref killing and RCU release */
-       struct rcu_head rcu_head;
        struct work_struct destroy_work;
+       struct rcu_work destroy_rwork;
 
        /*
         * PI: the parent css.  Placed here for cache proximity to following
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index bc0cda1..b39f3a4 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -13,6 +13,7 @@
 #include <linux/threads.h>
 #include <linux/atomic.h>
 #include <linux/cpumask.h>
+#include <linux/rcupdate.h>
 
 struct workqueue_struct;
 
@@ -120,6 +121,15 @@ struct delayed_work {
        int cpu;
 };
 
+struct rcu_work {
+       struct work_struct work;
+       struct rcu_head rcu;
+
+       /* target workqueue and CPU ->rcu uses to queue ->work */
+       struct workqueue_struct *wq;
+       int cpu;
+};
+
 /**
  * struct workqueue_attrs - A struct for workqueue attributes.
  *
@@ -151,6 +161,11 @@ static inline struct delayed_work *to_delayed_work(struct 
work_struct *work)
        return container_of(work, struct delayed_work, work);
 }
 
+static inline struct rcu_work *to_rcu_work(struct work_struct *work)
+{
+       return container_of(work, struct rcu_work, work);
+}
+
 struct execute_work {
        struct work_struct work;
 };
@@ -266,6 +281,12 @@ static inline unsigned int work_static(struct work_struct 
*work) { return 0; }
 #define INIT_DEFERRABLE_WORK_ONSTACK(_work, _func)                     \
        __INIT_DELAYED_WORK_ONSTACK(_work, _func, TIMER_DEFERRABLE)
 
+#define INIT_RCU_WORK(_work, _func)                                    \
+       INIT_WORK(&(_work)->work, (_func))
+
+#define INIT_RCU_WORK_ONSTACK(_work, _func)                            \
+       INIT_WORK_ONSTACK(&(_work)->work, (_func))
+
 /**
  * work_pending - Find out whether a work item is currently pending
  * @work: The work item in question
@@ -447,6 +468,8 @@ extern bool queue_delayed_work_on(int cpu, struct 
workqueue_struct *wq,
                        struct delayed_work *work, unsigned long delay);
 extern bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
                        struct delayed_work *dwork, unsigned long delay);
+extern bool queue_rcu_work_on(int cpu, struct workqueue_struct *wq,
+                       struct rcu_work *rwork);
 
 extern void flush_workqueue(struct workqueue_struct *wq);
 extern void drain_workqueue(struct workqueue_struct *wq);
@@ -463,6 +486,8 @@ extern bool flush_delayed_work(struct delayed_work *dwork);
 extern bool cancel_delayed_work(struct delayed_work *dwork);
 extern bool cancel_delayed_work_sync(struct delayed_work *dwork);
 
+extern bool flush_rcu_work(struct rcu_work *rwork);
+
 extern void workqueue_set_max_active(struct workqueue_struct *wq,
                                     int max_active);
 extern struct work_struct *current_work(void);
@@ -520,6 +545,19 @@ static inline bool mod_delayed_work(struct 
workqueue_struct *wq,
 }
 
 /**
+ * queue_rcu_work - queue work on a workqueue after a RCU grace period
+ * @wq: workqueue to use
+ * @rwork: RCU work to queue
+ *
+ * Equivalent to queue_rcu_work_on() but tries to use the local CPU.
+ */
+static inline bool queue_rcu_work(struct workqueue_struct *wq,
+                                 struct rcu_work *rwork)
+{
+       return queue_rcu_work_on(WORK_CPU_UNBOUND, wq, rwork);
+}
+
+/**
  * schedule_work_on - put work task on a specific cpu
  * @cpu: cpu to put the work task on
  * @work: job to be done
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8cda3bc..4c5d4ca0 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4514,10 +4514,10 @@ static struct cftype cgroup_base_files[] = {
  * and thus involve punting to css->destroy_work adding two additional
  * steps to the already complex sequence.
  */
-static void css_free_work_fn(struct work_struct *work)
+static void css_free_rwork_fn(struct work_struct *work)
 {
-       struct cgroup_subsys_state *css =
-               container_of(work, struct cgroup_subsys_state, destroy_work);
+       struct cgroup_subsys_state *css = container_of(to_rcu_work(work),
+                               struct cgroup_subsys_state, destroy_rwork);
        struct cgroup_subsys *ss = css->ss;
        struct cgroup *cgrp = css->cgroup;
 
@@ -4563,15 +4563,6 @@ static void css_free_work_fn(struct work_struct *work)
        }
 }
 
-static void css_free_rcu_fn(struct rcu_head *rcu_head)
-{
-       struct cgroup_subsys_state *css =
-               container_of(rcu_head, struct cgroup_subsys_state, rcu_head);
-
-       INIT_WORK(&css->destroy_work, css_free_work_fn);
-       queue_work(cgroup_destroy_wq, &css->destroy_work);
-}
-
 static void css_release_work_fn(struct work_struct *work)
 {
        struct cgroup_subsys_state *css =
@@ -4621,7 +4612,8 @@ static void css_release_work_fn(struct work_struct *work)
 
        mutex_unlock(&cgroup_mutex);
 
-       call_rcu(&css->rcu_head, css_free_rcu_fn);
+       INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
+       queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 }
 
 static void css_release(struct percpu_ref *ref)
@@ -4755,7 +4747,8 @@ static struct cgroup_subsys_state *css_create(struct 
cgroup *cgrp,
 err_list_del:
        list_del_rcu(&css->sibling);
 err_free_css:
-       call_rcu(&css->rcu_head, css_free_rcu_fn);
+       INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
+       queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
        return ERR_PTR(err);
 }
 
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index bb9a519..e26c2f4 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1604,6 +1604,40 @@ bool mod_delayed_work_on(int cpu, struct 
workqueue_struct *wq,
 }
 EXPORT_SYMBOL_GPL(mod_delayed_work_on);
 
+static void rcu_work_rcufn(struct rcu_head *rcu)
+{
+       struct rcu_work *rwork = container_of(rcu, struct rcu_work, rcu);
+
+       /* read the comment in __queue_work() */
+       local_irq_disable();
+       __queue_work(rwork->cpu, rwork->wq, &rwork->work);
+       local_irq_enable();
+}
+
+/**
+ * queue_rcu_work_on - queue work on specific CPU after a RCU grace period
+ * @cpu: CPU number to execute work on
+ * @wq: workqueue to use
+ * @rwork: work to queue
+ *
+ * Return: %false if @work was already on a queue, %true otherwise.
+ */
+bool queue_rcu_work_on(int cpu, struct workqueue_struct *wq,
+                      struct rcu_work *rwork)
+{
+       struct work_struct *work = &rwork->work;
+
+       if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
+               rwork->wq = wq;
+               rwork->cpu = cpu;
+               call_rcu(&rwork->rcu, rcu_work_rcufn);
+               return true;
+       }
+
+       return false;
+}
+EXPORT_SYMBOL(queue_rcu_work_on);
+
 /**
  * worker_enter_idle - enter idle state
  * @worker: worker which is entering idle state
@@ -3001,6 +3035,26 @@ bool flush_delayed_work(struct delayed_work *dwork)
 }
 EXPORT_SYMBOL(flush_delayed_work);
 
+/**
+ * flush_rcu_work - wait for a rwork to finish executing the last queueing
+ * @rwork: the rcu work to flush
+ *
+ * Return:
+ * %true if flush_rcu_work() waited for the work to finish execution,
+ * %false if it was already idle.
+ */
+bool flush_rcu_work(struct rcu_work *rwork)
+{
+       if (test_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&rwork->work))) {
+               rcu_barrier();
+               flush_work(&rwork->work);
+               return true;
+       } else {
+               return flush_work(&rwork->work);
+       }
+}
+EXPORT_SYMBOL(flush_rcu_work);
+
 static bool __cancel_work(struct work_struct *work, bool is_dwork)
 {
        unsigned long flags;
-- 
2.9.5

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