On Wed, Sep 14, 2011 at 04:53:38PM -0700, Valerie Aurora wrote:

On Wed, Sep 14, 2011 at 7:00 AM, Jan Kara [off-list ref] wrote:

quoted

On Mon 12-09-11 19:57:11, Valerie Aurora wrote:

quoted

Val, if you are sending patches as attachments, make them at least
text/plain please!

Oops, sorry.

quoted

quoted

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 04cf3b9..d1dca03 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c

@@ -537,6 +537,9 @@ static long writeback_sb_inodes(struct super_block *sb,

      long write_chunk;
      long wrote = 0;  /* count both pages and inodes */

+     if (vfs_is_frozen(sb))
+             return 0;
+

 Umm, maybe we could make this more robust by skipping the superblock in
__writeback_inodes_wb() and just explicitely stopping the writeback when
work->sb is set (i.e. writeback is required only for frozen sb) in
wb_writeback()?

Sorry, I don't quite understand what the goal is here?  I'm happy to
make the change, just want to make sure I'm accomplishing what you
want.

quoted

quoted

      while (!list_empty(&wb->b_io)) {
              struct inode *inode = wb_inode(wb->b_io.prev);

@@ -1238,39 +1241,43 @@ EXPORT_SYMBOL(writeback_inodes_sb);

  * writeback_inodes_sb_if_idle       -       start writeback if none underway
  * @sb: the superblock
  *
- * Invoke writeback_inodes_sb if no writeback is currently underway.
- * Returns 1 if writeback was started, 0 if not.
+ * Invoke writeback_inodes_sb if no writeback is currently underway
+ * and no one else holds the s_umount lock.  Returns 1 if writeback
+ * was started, 0 if not.
  */
 int writeback_inodes_sb_if_idle(struct super_block *sb)
 {
      if (!writeback_in_progress(sb->s_bdi)) {
-             down_read(&sb->s_umount);
-             writeback_inodes_sb(sb);
-             up_read(&sb->s_umount);
-             return 1;
-     } else
-             return 0;
+             if (down_read_trylock(&sb->s_umount)) {

 What's exactly the deadlock trylock protects from here? Or is it just an
optimization?

The trylock is an optimization Dave Chinner suggested.  The first
version I wrote acquired the lock and then checked vfs_is_frozen().

It's not so much an optimisation, but the general case of avoiding
read-write deadlocks such that freezing can trigger. I think remount
can trigger the same deadlock as freezing, so the trylock avoids both
deadlock cases rather than just working around the freeze problem....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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