In the transaction commit path we are acquiring the tree log mutex too
early and we have a stale comment because:
1) It mentions a function named btrfs_commit_tree_roots(), which does not
exists anymore, it was the old name of commit_cowonly_roots(), renamed
a very long time ago by commit 5d4f98a28c ("Btrfs: Mixed back
reference (FORWARD ROLLING FORMAT CHANGE)"));
2) It mentions that we need to acquire the tree log mutex at that point
to ensure we have no running log writers. That is not correct anymore,
for many years at least, since we are guaranteed that we do not have
any log writers at that point simply because we have set the state of
the transaction to TRANS_STATE_COMMIT_DOING and have waited for all
writers to complete - meaning no one can log until we change the state
of the transaction to TRANS_STATE_UNBLOCKED. Any attempts to join the
transaction or start a new one will block until we do that state
transition;
3) The comment mentions a "trans mutex" which doesn't exists since 2011,
commit a4abeea41a ("Btrfs: kill trans_mutex") removed it;
4) The current use of the tree log mutex is to ensure proper serialization
of super block writes - if someone started a new transaction and uses it
for logging, it will wait for the previous transaction to write its
super block before writing the super block when attempting to sync the
log.
So acquire the tree log mutex only when it's absolutely needed, before
setting the transaction state to TRANS_STATE_UNBLOCKED, fix and move the
stale comment, add some assertions and new comments where appropriate.
Also, this has no effect on concurrency or performance, since the new
start of the critical section is still when the transaction is in the
state TRANS_STATE_COMMIT_DOING.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
dfba78dc1c