writeback: use RCU to protect bdi_list

Now that bdi_writeback_all() no longer handles integrity writeback,
it doesn't have to block anymore. This means that we can switch
bdi_list reader side protection to RCU.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

4 files changed, 63 insertions, 28 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 14f06b459197..f8cd7a97f5b7 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -868,16 +868,16 @@ static void bdi_writeback_all(struct writeback_control *wbc)
 
         WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
 
-        spin_lock(&bdi_lock);
+        rcu_read_lock();
 
-        list_for_each_entry(bdi, &bdi_list, bdi_list) {
+        list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
                 if (!bdi_has_dirty_io(bdi))
                         continue;
 
                 bdi_alloc_queue_work(bdi, wbc);
         }
 
-        spin_unlock(&bdi_lock);
+        rcu_read_unlock();
 }
 
 /*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index f169bcb90b58..859e797f4576 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -59,6 +59,7 @@ struct bdi_writeback {
 
 struct backing_dev_info {
         struct list_head bdi_list;
+        struct rcu_head rcu_head;
         unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
         unsigned long state;    /* Always use atomic bitops on this */
         unsigned int capabilities; /* Device capabilities */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d3ca0dac1111..fd93566345b6 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -26,6 +26,12 @@ struct backing_dev_info default_backing_dev_info = {
 EXPORT_SYMBOL_GPL(default_backing_dev_info);
 
 static struct class *bdi_class;
+
+/*
+ * bdi_lock protects updates to bdi_list and bdi_pending_list, as well as
+ * reader side protection for bdi_pending_list. bdi_list has RCU reader side
+ * locking.
+ */
 DEFINE_SPINLOCK(bdi_lock);
 LIST_HEAD(bdi_list);
 LIST_HEAD(bdi_pending_list);
@@ -284,9 +290,9 @@ static int bdi_start_fn(void *ptr)
         /*
          * Add us to the active bdi_list
          */
-        spin_lock(&bdi_lock);
-        list_add(&bdi->bdi_list, &bdi_list);
-        spin_unlock(&bdi_lock);
+        spin_lock_bh(&bdi_lock);
+        list_add_rcu(&bdi->bdi_list, &bdi_list);
+        spin_unlock_bh(&bdi_lock);
 
         bdi_task_init(bdi, wb);
 
@@ -389,7 +395,7 @@ static int bdi_forker_task(void *ptr)
                 if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
                         wb_do_writeback(me, 0);
 
-                spin_lock(&bdi_lock);
+                spin_lock_bh(&bdi_lock);
 
                 /*
                  * Check if any existing bdi's have dirty data without
@@ -410,7 +416,7 @@ static int bdi_forker_task(void *ptr)
                 if (list_empty(&bdi_pending_list)) {
                         unsigned long wait;
 
-                        spin_unlock(&bdi_lock);
+                        spin_unlock_bh(&bdi_lock);
                         wait = msecs_to_jiffies(dirty_writeback_interval * 10);
                         schedule_timeout(wait);
                         try_to_freeze();
@@ -426,7 +432,7 @@ static int bdi_forker_task(void *ptr)
                 bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
                                  bdi_list);
                 list_del_init(&bdi->bdi_list);
-                spin_unlock(&bdi_lock);
+                spin_unlock_bh(&bdi_lock);
 
                 wb = &bdi->wb;
                 wb->task = kthread_run(bdi_start_fn, wb, "flush-%s",
@@ -445,9 +451,9 @@ static int bdi_forker_task(void *ptr)
                          * a chance to flush other bdi's to free
                          * memory.
                          */
-                        spin_lock(&bdi_lock);
+                        spin_lock_bh(&bdi_lock);
                         list_add_tail(&bdi->bdi_list, &bdi_pending_list);
-                        spin_unlock(&bdi_lock);
+                        spin_unlock_bh(&bdi_lock);
 
                         bdi_flush_io(bdi);
                 }
@@ -456,6 +462,24 @@ static int bdi_forker_task(void *ptr)
         return 0;
 }
 
+static void bdi_add_to_pending(struct rcu_head *head)
+{
+        struct backing_dev_info *bdi;
+
+        bdi = container_of(head, struct backing_dev_info, rcu_head);
+        INIT_LIST_HEAD(&bdi->bdi_list);
+
+        spin_lock(&bdi_lock);
+        list_add_tail(&bdi->bdi_list, &bdi_pending_list);
+        spin_unlock(&bdi_lock);
+
+        /*
+         * We are now on the pending list, wake up bdi_forker_task()
+         * to finish the job and add us back to the active bdi_list
+         */
+        wake_up_process(default_backing_dev_info.wb.task);
+}
+
 /*
  * Add the default flusher task that gets created for any bdi
  * that has dirty data pending writeout
@@ -478,16 +502,29 @@ void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
          * waiting for previous additions to finish.
          */
         if (!test_and_set_bit(BDI_pending, &bdi->state)) {
-                list_move_tail(&bdi->bdi_list, &bdi_pending_list);
+                list_del_rcu(&bdi->bdi_list);
 
                 /*
-                 * We are now on the pending list, wake up bdi_forker_task()
-                 * to finish the job and add us back to the active bdi_list
+                 * We must wait for the current RCU period to end before
+                 * moving to the pending list. So schedule that operation
+                 * from an RCU callback.
                  */
-                wake_up_process(default_backing_dev_info.wb.task);
+                call_rcu(&bdi->rcu_head, bdi_add_to_pending);
         }
 }
 
+/*
+ * Remove bdi from bdi_list, and ensure that it is no longer visible
+ */
+static void bdi_remove_from_list(struct backing_dev_info *bdi)
+{
+        spin_lock_bh(&bdi_lock);
+        list_del_rcu(&bdi->bdi_list);
+        spin_unlock_bh(&bdi_lock);
+
+        synchronize_rcu();
+}
+
 int bdi_register(struct backing_dev_info *bdi, struct device *parent,
                 const char *fmt, ...)
 {
@@ -506,9 +543,9 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
                 goto exit;
         }
 
-        spin_lock(&bdi_lock);
-        list_add_tail(&bdi->bdi_list, &bdi_list);
-        spin_unlock(&bdi_lock);
+        spin_lock_bh(&bdi_lock);
+        list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
+        spin_unlock_bh(&bdi_lock);
 
         bdi->dev = dev;
 
@@ -526,9 +563,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
                         wb->task = NULL;
                         ret = -ENOMEM;
 
-                        spin_lock(&bdi_lock);
-                        list_del(&bdi->bdi_list);
-                        spin_unlock(&bdi_lock);
+                        bdi_remove_from_list(bdi);
                         goto exit;
                 }
         }
@@ -565,9 +600,7 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
         /*
          * Make sure nobody finds us on the bdi_list anymore
          */
-        spin_lock(&bdi_lock);
-        list_del(&bdi->bdi_list);
-        spin_unlock(&bdi_lock);
+        bdi_remove_from_list(bdi);
 
         /*
          * Finally, kill the kernel threads. We don't need to be RCU
@@ -599,6 +632,7 @@ int bdi_init(struct backing_dev_info *bdi)
         bdi->max_ratio = 100;
         bdi->max_prop_frac = PROP_FRAC_BASE;
         spin_lock_init(&bdi->wb_lock);
+        INIT_RCU_HEAD(&bdi->rcu_head);
         INIT_LIST_HEAD(&bdi->bdi_list);
         INIT_LIST_HEAD(&bdi->wb_list);
         INIT_LIST_HEAD(&bdi->work_list);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index abc648f5de00..12c3d843ce93 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -315,7 +315,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
         int ret = 0;
 
-        spin_lock(&bdi_lock);
+        spin_lock_bh(&bdi_lock);
         if (min_ratio > bdi->max_ratio) {
                 ret = -EINVAL;
         } else {
@@ -327,7 +327,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
                         ret = -EINVAL;
                 }
         }
-        spin_unlock(&bdi_lock);
+        spin_unlock_bh(&bdi_lock);
 
         return ret;
 }
@@ -339,14 +339,14 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
         if (max_ratio > 100)
                 return -EINVAL;
 
-        spin_lock(&bdi_lock);
+        spin_lock_bh(&bdi_lock);
         if (bdi->min_ratio > max_ratio) {
                 ret = -EINVAL;
         } else {
                 bdi->max_ratio = max_ratio;
                 bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100;
         }
-        spin_unlock(&bdi_lock);
+        spin_unlock_bh(&bdi_lock);
 
         return ret;
 }