linux-kernel - Re: [PATCH RFC 7/7] libfs: Re-arrange locking in offset_iterate

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Date: Mon, 19 Feb 2024 19:06:41 +0100
From: Jan Kara <jack@...e.cz>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Jan Kara <jack@...e.cz>, Chuck Lever <cel@...nel.org>,
	viro@...iv.linux.org.uk, brauner@...nel.org, hughd@...gle.com,
	akpm@...ux-foundation.org, oliver.sang@...el.com,
	feng.tang@...el.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, maple-tree@...ts.infradead.org,
	linux-mm@...ck.org, lkp@...el.com
Subject: Re: [PATCH RFC 7/7] libfs: Re-arrange locking in offset_iterate_dir()

On Fri 16-02-24 11:33:18, Liam R. Howlett wrote:
> * Jan Kara <jack@...e.cz> [240216 05:15]:
> > > If you have other examples you think are unsafe then I can have a look
> > > at them as well.
> > 
> > I'm currently not aware of any but I'll let you know if I find some.
> > Missing xas/mas_pause() seems really easy.
> 
> What if we convert the rcu_read_lock() to a mas_read_lock() or
> xas_read_lock() and we can check to ensure the state isn't being locked
> without being in the 'parked' state (paused or otherwise)?
> 
> mas_read_lock(struct ma_state *mas) {
> 	assert(!mas_active(mas));
> 	rcu_read_lock();
> }
> 
> Would that be a reasonable resolution to your concern?  Unfortunately,
> what was done with the locking in this case would not be detected with
> this change unless the rcu_read_lock() was replaced.  IOW, people could
> still use the rcu_read_lock() and skip the detection.

Yes, I guess this is still better than nothing.

> Doing the same in the mas_unlock() doesn't make as much sense since that
> may be called without the intent to reuse the state at all.  So we'd be
> doing more work than necessary at the end of each loop or use.

Yes, understood.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR