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>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 14f06b45919749f3ac06f31138279baaae1832b4..f8cd7a97f5b7671c20dcbfc26d992eba831b0134 100644 (file)
--- 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 f169bcb90b58b3b098208984a50a921132bd9f5f..859e797f4576142922ee6552534dcd2d810bce90 100644 (file)
--- 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 d3ca0dac1111ed4f22cba933acf535a85f43b3aa..fd93566345b6ade36f527c14c2bdc0c6f60d243b 100644 (file)
--- 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 abc648f5de00616fc2eb20b33ba9675bf3b43bd5..12c3d843ce93106acf23a5139243ec3a8dccd33c 100644 (file)
--- 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;
 }