writeback: restructure try_writeback_inodes_sb[_nr]()

try_writeback_inodes_sb_nr() wraps writeback_inodes_sb_nr() so that it
handles s_umount locking and skips if writeback is already in
progress.  The in progress test is performed on the root wb
(bdi_writeback) which isn't sufficient for cgroup writeback support.
The test must be done per-wb.

To prepare for the change, this patch factors out
__writeback_inodes_sb_nr() from writeback_inodes_sb_nr() and adds
@skip_if_busy and moves the in progress test right before queueing the
wb_writeback_work.  try_writeback_inodes_sb_nr() now just grabs
s_umount and invokes __writeback_inodes_sb_nr() with asserted
@skip_if_busy.  This way, later addition of multiple wb handling can
skip only the wb's which already have writeback in progress.

This swaps the order between in progress test and s_umount test which
can flip the return value when writeback is in progress and s_umount
is being held by someone else but this shouldn't cause any meaningful
difference.  It's a fringe condition and the return value is an
unsynchronized hint anyway.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index a6b9db7fcee8..23af355d5471 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -93,9 +93,9 @@ struct bdi_writeback;
 void writeback_inodes_sb(struct super_block *, enum wb_reason reason);
 void writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
 							enum wb_reason reason);
-int try_to_writeback_inodes_sb(struct super_block *, enum wb_reason reason);
-int try_to_writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
-				  enum wb_reason reason);
+bool try_to_writeback_inodes_sb(struct super_block *, enum wb_reason reason);
+bool try_to_writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
+				   enum wb_reason reason);
 void sync_inodes_sb(struct super_block *);
 void wakeup_flusher_threads(long nr_pages, enum wb_reason reason);
 void inode_wait_for_writeback(struct inode *inode);