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From: David Sterba <dsterba@suse.cz>
To: Nikolay Borisov <nborisov@suse.com>
Cc: dsterba@suse.cz, Josef Bacik <josef@toxicpanda.com>,
	linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit
Date: Mon, 11 Jan 2021 22:50:51 +0100	[thread overview]
Message-ID: <20210111215051.GH6430@twin.jikos.cz> (raw)
In-Reply-To: <52aef9a6-efc7-0820-7056-067e69c2a856@suse.com>

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?

  reply	other threads:[~2021-01-11 21:53 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-18 19:24 [PATCH v5 0/8] A variety of lock contention fixes Josef Bacik
2020-12-18 19:24 ` [PATCH v5 1/8] btrfs: do not block on deleted bgs mutex in the cleaner Josef Bacik
2020-12-18 19:24 ` [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit Josef Bacik
2021-01-08 16:01   ` David Sterba
2021-01-11  8:33     ` Nikolay Borisov
2021-01-11 21:50       ` David Sterba [this message]
2021-01-12  9:17         ` Nikolay Borisov
2021-01-26 17:36           ` David Sterba
2020-12-18 19:24 ` [PATCH v5 3/8] btrfs: delayed refs pre-flushing should only run the heads we have Josef Bacik
2020-12-18 19:24 ` [PATCH v5 4/8] btrfs: only run delayed refs once before committing Josef Bacik
2020-12-18 19:24 ` [PATCH v5 5/8] btrfs: move delayed ref flushing for qgroup into qgroup helper Josef Bacik
2020-12-18 19:24 ` [PATCH v5 6/8] btrfs: remove bogus BUG_ON in alloc_reserved_tree_block Josef Bacik
2020-12-18 19:24 ` [PATCH v5 7/8] btrfs: stop running all delayed refs during snapshot Josef Bacik
2020-12-18 19:24 ` [PATCH v5 8/8] btrfs: run delayed refs less often in commit_cowonly_roots Josef Bacik
2021-01-26 17:51 ` [PATCH v5 0/8] A variety of lock contention fixes David Sterba

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