Btrfs: fix deadlock on async thread startup

The btrfs async worker threads are used for a wide variety of things,
including processing bio end_io functions.  This means that when
the endio threads aren't running, the rest of the FS isn't
able to do the final processing required to clear PageWriteback.

The endio threads also try to exit as they become idle and
start more as the work piles up.  The problem is that starting more
threads means kthreadd may need to allocate ram, and that allocation
may wait until the global number of writeback pages on the system is
below a certain limit.

The result of that throttling is that end IO threads wait on
kthreadd, who is waiting on IO to end, which will never happen.

This commit fixes the deadlock by handing off thread startup to a
dedicated thread.  It also fixes a bug where the on-demand thread
creation was creating far too many threads because it didn't take into
account threads being started by other procs.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 23 insertions, 19 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 69dce50aabd2..9903f042765d 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1746,21 +1746,22 @@ struct btrfs_root *open_ctree(struct super_block *sb,
                 err = -EINVAL;
                 goto fail_iput;
         }
-printk("thread pool is %d\n", fs_info->thread_pool_size);
-        /*
-         * we need to start all the end_io workers up front because the
-         * queue work function gets called at interrupt time, and so it
-         * cannot dynamically grow.
-         */
+
+        btrfs_init_workers(&fs_info->generic_worker,
+                           "genwork", 1, NULL);
+
         btrfs_init_workers(&fs_info->workers, "worker",
-                           fs_info->thread_pool_size);
+                           fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
 
         btrfs_init_workers(&fs_info->delalloc_workers, "delalloc",
-                           fs_info->thread_pool_size);
+                           fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
 
         btrfs_init_workers(&fs_info->submit_workers, "submit",
                            min_t(u64, fs_devices->num_devices,
-                           fs_info->thread_pool_size));
+                           fs_info->thread_pool_size),
+                           &fs_info->generic_worker);
 
         /* a higher idle thresh on the submit workers makes it much more
          * likely that bios will be send down in a sane order to the
@@ -1774,15 +1775,20 @@ printk("thread pool is %d\n", fs_info->thread_pool_size);
         fs_info->delalloc_workers.idle_thresh = 2;
         fs_info->delalloc_workers.ordered = 1;
 
-        btrfs_init_workers(&fs_info->fixup_workers, "fixup", 1);
+        btrfs_init_workers(&fs_info->fixup_workers, "fixup", 1,
+                           &fs_info->generic_worker);
         btrfs_init_workers(&fs_info->endio_workers, "endio",
-                           fs_info->thread_pool_size);
+                           fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
         btrfs_init_workers(&fs_info->endio_meta_workers, "endio-meta",
-                           fs_info->thread_pool_size);
+                           fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
         btrfs_init_workers(&fs_info->endio_meta_write_workers,
-                           "endio-meta-write", fs_info->thread_pool_size);
+                           "endio-meta-write", fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
         btrfs_init_workers(&fs_info->endio_write_workers, "endio-write",
-                           fs_info->thread_pool_size);
+                           fs_info->thread_pool_size,
+                           &fs_info->generic_worker);
 
         /*
          * endios are largely parallel and should have a very
@@ -1794,12 +1800,8 @@ printk("thread pool is %d\n", fs_info->thread_pool_size);
         fs_info->endio_write_workers.idle_thresh = 2;
         fs_info->endio_meta_write_workers.idle_thresh = 2;
 
-        fs_info->endio_workers.atomic_worker_start = 1;
-        fs_info->endio_meta_workers.atomic_worker_start = 1;
-        fs_info->endio_write_workers.atomic_worker_start = 1;
-        fs_info->endio_meta_write_workers.atomic_worker_start = 1;
-
         btrfs_start_workers(&fs_info->workers, 1);
+        btrfs_start_workers(&fs_info->generic_worker, 1);
         btrfs_start_workers(&fs_info->submit_workers, 1);
         btrfs_start_workers(&fs_info->delalloc_workers, 1);
         btrfs_start_workers(&fs_info->fixup_workers, 1);
@@ -2012,6 +2014,7 @@ fail_chunk_root:
         free_extent_buffer(chunk_root->node);
         free_extent_buffer(chunk_root->commit_root);
 fail_sb_buffer:
+        btrfs_stop_workers(&fs_info->generic_worker);
         btrfs_stop_workers(&fs_info->fixup_workers);
         btrfs_stop_workers(&fs_info->delalloc_workers);
         btrfs_stop_workers(&fs_info->workers);
@@ -2437,6 +2440,7 @@ int close_ctree(struct btrfs_root *root)
 
         iput(fs_info->btree_inode);
 
+        btrfs_stop_workers(&fs_info->generic_worker);
         btrfs_stop_workers(&fs_info->fixup_workers);
         btrfs_stop_workers(&fs_info->delalloc_workers);
         btrfs_stop_workers(&fs_info->workers);