On Fri, 2012-01-06 at 01:35 +0100, Jan Kara wrote:

On Thu 08-12-11 10:04:35, Kamal Mostafa wrote:

quoted

From: Valerie Aurora <redacted>

Use trylock in try_to_writeback_inodes_sb to avoid read-write
deadlocks that could be triggered by freeze.

  Christoph asked about what is the exact deadlock this patch tries to fix.
I don't think you answered that. So can you elaborate please? Is it somehow
connected with the fact that ext4 calls try_to_writeback_inodes_sb() with
i_mutex held?

								Honza

This was discussed in the thread
http://www.spinics.net/lists/linux-fsdevel/msg48754.html
Summarizing...

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

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

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

 -Kamal

quoted

BugLink: https://bugs.launchpad.net/bugs/897421
Signed-off-by: Valerie Aurora <redacted>
Cc: Kamal Mostafa <redacted>
Tested-by: Peter M. Petrakis <redacted>
[kamal@canonical.com: patch restructure]
Signed-off-by: Kamal Mostafa <redacted>
---
 fs/fs-writeback.c |   13 ++++++++-----
 1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index ea89b3f..3a80f1b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c

@@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb);
  * try_to_writeback_inodes_sb	-	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 try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
 {

@@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb);
  * @sb: the superblock
  * @nr: the number of pages to write
  *
- * 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 try_to_writeback_inodes_sb_nr(struct super_block *sb,
 				   unsigned long nr,
 				   enum wb_reason reason)
 {
 	if (!writeback_in_progress(sb->s_bdi)) {
-		down_read(&sb->s_umount);
+		if (!down_read_trylock(&sb->s_umount))
+		    return 0;
 		if (nr == 0)
 			writeback_inodes_sb(sb, reason);
 		else

-- 
1.7.5.4