On Mon, Apr 11, 2016 at 05:37:40PM -0400, Josef Bacik wrote:
> Using the offwakecputime bpf script I noticed most of our time was spent waiting
> on the delayed ref throttling. This is what is supposed to happen, but
> sometimes the transaction can commit and then we're waiting for throttling that
> doesn't matter anymore. So change this stuff to be a little smarter by tracking
> the transid we were in when we initiated the throttling. If the transaction we
> get is different then we can just bail out. This resulted in a 50% speedup in
> my fs_mark test, and reduced the amount of time spent throttling by 60 seconds
> over the entire run (which is about 30 minutes). Thanks,
>
> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> ---
> fs/btrfs/ctree.h | 2 +-
> fs/btrfs/extent-tree.c | 15 ++++++++++++---
> fs/btrfs/inode.c | 1 +
> fs/btrfs/transaction.c | 3 ++-
> 4 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 55a24c5..4222936 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3505,7 +3505,7 @@ void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
> int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
> struct btrfs_root *root, unsigned long count);
> int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> - unsigned long count, int wait);
> + unsigned long count, u64 transid, int wait);
> int btrfs_lookup_data_extent(struct btrfs_root *root, u64 start, u64 len);
> int btrfs_lookup_extent_info(struct btrfs_trans_handle *trans,
> struct btrfs_root *root, u64 bytenr,
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 4b5a517..f23f426 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2839,6 +2839,7 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
>
> struct async_delayed_refs {
> struct btrfs_root *root;
> + u64 transid;
> int count;
> int error;
> int sync;
> @@ -2854,9 +2855,16 @@ static void delayed_ref_async_start(struct btrfs_work *work)
>
> async = container_of(work, struct async_delayed_refs, work);
>
> - trans = btrfs_join_transaction(async->root);
> + trans = btrfs_attach_transaction(async->root);
> if (IS_ERR(trans)) {
> - async->error = PTR_ERR(trans);
> + if (PTR_ERR(trans) != -ENOENT)
> + async->error = PTR_ERR(trans);
> + goto done;
> + }
This ends up deadlocking because btrfs_attach_transaction waits in ways
that join does not. The differences between these two are really
subtle, and we manage to make this mistake every year or so.
Subject: [PATCH] btrfs: fix deadlock in delayed_ref_async_start
"Btrfs: track transid for delayed ref flushing" was deadlocking on
btrfs_attach_transaction because its not safe to call from the async
delayed ref start code. This commit brings back btrfs_join_transaction
instead and checks for a blocked commit.
Signed-off-by: Josef Bacik <jbacik@xxxxxx>
Signed-off-by: Chris Mason <clm@xxxxxx>
---
fs/btrfs/extent-tree.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 6ce5b6c..44da4ac 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2845,16 +2845,13 @@ static void delayed_ref_async_start(struct btrfs_work *work)
async = container_of(work, struct async_delayed_refs, work);
- trans = btrfs_attach_transaction(async->root);
- if (IS_ERR(trans)) {
- if (PTR_ERR(trans) != -ENOENT)
- async->error = PTR_ERR(trans);
+ /* if the commit is already started, we don't need to wait here */
+ if (btrfs_transaction_blocked(async->root->fs_info))
goto done;
- }
- /* Don't bother flushing if we got into a different transaction */
- if (trans->transid != async->transid) {
- btrfs_end_transaction(trans, async->root);
+ trans = btrfs_join_transaction(async->root);
+ if (IS_ERR(trans)) {
+ async->error = PTR_ERR(trans);
goto done;
}
@@ -2863,10 +2860,15 @@ static void delayed_ref_async_start(struct btrfs_work *work)
* wait on delayed refs
*/
trans->sync = true;
+
+ /* Don't bother flushing if we got into a different transaction */
+ if (trans->transid > async->transid)
+ goto end;
+
ret = btrfs_run_delayed_refs(trans, async->root, async->count);
if (ret)
async->error = ret;
-
+end:
ret = btrfs_end_transaction(trans, async->root);
if (ret && !async->error)
async->error = ret;
--
2.8.0.rc2
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