Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit

[David Sterba <dsterba@suse.cz>]

On Mon, Jan 11, 2021 at 10:33:42AM +0200, Nikolay Borisov wrote:
> On 8.01.21 г. 18:01 ч., David Sterba wrote:
> > On Fri, Dec 18, 2020 at 02:24:20PM -0500, Josef Bacik wrote:
> >> @@ -2043,23 +2043,22 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
> >>  	btrfs_trans_release_metadata(trans);
> >>  	trans->block_rsv = NULL;
> >>  
> >> -	/* make a pass through all the delayed refs we have so far
> >> -	 * any runnings procs may add more while we are here
> >> -	 */
> >> -	ret = btrfs_run_delayed_refs(trans, 0);
> >> -	if (ret) {
> >> -		btrfs_end_transaction(trans);
> >> -		return ret;
> >> -	}
> >> -
> >> -	cur_trans = trans->transaction;
> >> -
> >>  	/*
> >> -	 * set the flushing flag so procs in this transaction have to
> >> -	 * start sending their work down.
> >> +	 * We only want one transaction commit doing the flushing so we do not
> >> +	 * waste a bunch of time on lock contention on the extent root node.
> >>  	 */
> >> -	cur_trans->delayed_refs.flushing = 1;
> >> -	smp_wmb();
> > 
> > This barrier obviously separates the flushing = 1 and the rest of the
> > code, now implemented as test_and_set_bit, which implies full barrier.
> > 
> > However, hunk in btrfs_should_end_transaction removes the barrier and
> > I'm not sure whether this is correct:
> > 
> > -	smp_mb();
> >  	if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
> > -	    cur_trans->delayed_refs.flushing)
> > +	    test_bit(BTRFS_DELAYED_REFS_FLUSHING,
> > +		     &cur_trans->delayed_refs.flags))
> >  		return true;
> > 
> > This is never called under locks so we don't have complete
> > synchronization of neither the transaction state nor the flushing bit.
> > btrfs_should_end_transaction is merely a hint and not called in critical
> > places so we could probably afford to keep it without a barrier, or keep
> > it with comment(s).
> 
> I think the point is moot in this case, because the test_bit either sees
> the flag or it doesn't. It's not possible for the flag to be set AND
> should_end_transaction return false that would be gross violation of
> program correctness.

So that's for the flushing part, but what about cur_trans->state?