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btrfs: transaction: describe transaction states and transitions

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Add an overview of the basic btrfs transaction transitions, including
the following states:

- No transaction states
- Transaction N [[TRANS_STATE_RUNNING]]
- Transaction N [[TRANS_STATE_COMMIT_START]]
- Transaction N [[TRANS_STATE_COMMIT_DOING]]
- Transaction N [[TRANS_STATE_UNBLOCKED]]
- Transaction N [[TRANS_STATE_COMPLETED]]

For each state, the comment will include:

- Basic explaination about current state
- How to go next stage
- What will happen if we call various start_transaction() functions
- Relationship to transaction N+1

This doesn't provide tech details, but serves as a cheat sheet for
reader to get into the code a little easier.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2019-08-22 15:24:59 +08:00 committed by David Sterba
parent c1499166d1
commit 61c047b541
1 changed files with 71 additions and 0 deletions
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71
fs/btrfs/transaction.c
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@ -24,6 +24,77 @@
#define BTRFS_ROOT_TRANS_TAG 0
/*
* Transaction states and transitions
*
* No running transaction (fs tree blocks are not modified)
* |
* | To next stage:
* | Call start_transaction() variants. Except btrfs_join_transaction_nostart().
* V
* Transaction N [[TRANS_STATE_RUNNING]]
* |
* | New trans handles can be attached to transaction N by calling all
* | start_transaction() variants.
* |
* | To next stage:
* | Call btrfs_commit_transaction() on any trans handle attached to
* | transaction N
* V
* Transaction N [[TRANS_STATE_COMMIT_START]]
* |
* | Will wait for previous running transaction to completely finish if there
* | is one
* |
* | Then one of the following happes:
* | - Wait for all other trans handle holders to release.
* | The btrfs_commit_transaction() caller will do the commit work.
* | - Wait for current transaction to be committed by others.
* | Other btrfs_commit_transaction() caller will do the commit work.
* |
* | At this stage, only btrfs_join_transaction*() variants can attach
* | to this running transaction.
* | All other variants will wait for current one to finish and attach to
* | transaction N+1.
* |
* | To next stage:
* | Caller is chosen to commit transaction N, and all other trans handle
* | haven been released.
* V
* Transaction N [[TRANS_STATE_COMMIT_DOING]]
* |
* | The heavy lifting transaction work is started.
* | From running delayed refs (modifying extent tree) to creating pending
* | snapshots, running qgroups.
* | In short, modify supporting trees to reflect modifications of subvolume
* | trees.
* |
* | At this stage, all start_transaction() calls will wait for this
* | transaction to finish and attach to transaction N+1.
* |
* | To next stage:
* | Until all supporting trees are updated.
* V
* Transaction N [[TRANS_STATE_UNBLOCKED]]
* | Transaction N+1
* | All needed trees are modified, thus we only [[TRANS_STATE_RUNNING]]
* | need to write them back to disk and update |
* | super blocks. |
* | |
* | At this stage, new transaction is allowed to |
* | start. |
* | All new start_transaction() calls will be |
* | attached to transid N+1. |
* | |
* | To next stage: |
* | Until all tree blocks are super blocks are |
* | written to block devices |
* V |
* Transaction N [[TRANS_STATE_COMPLETED]] V
* All tree blocks and super blocks are written. Transaction N+1
* This transaction is finished and all its [[TRANS_STATE_COMMIT_START]]
* data structures will be cleaned up. | Life goes on
*/
static const unsigned int btrfs_blocked_trans_types[TRANS_STATE_MAX] = {
[TRANS_STATE_RUNNING] = 0U,
[TRANS_STATE_BLOCKED] = __TRANS_START,