linux-ext4 - Re: [PATCH 3/6] vfs: Allow searching of the icache under RCU conditions [ver #2]

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Date:   Thu, 25 Apr 2019 16:45:27 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, linux-afs@...ts.infradead.org,
        linux-ext4@...r.kernel.org, linux-ntfs-dev@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] vfs: Allow searching of the icache under RCU conditions [ver #2]

Al Viro <viro@...iv.linux.org.uk> wrote:

> Hmm...  Why do these stores to ->i_state need WRITE_ONCE, while an arseload
> of similar in fs/fs-writeback.c does not?

Because what matters in find_inode_rcu() are the I_WILL_FREE and I_FREEING
flags - and there's a gap during iput_final() where neither is set.

	if (!drop) {
		inode->i_state |= I_WILL_FREE;
		spin_unlock(&inode->i_lock);
		write_inode_now(inode, 1);
		spin_lock(&inode->i_lock);
		WARN_ON(inode->i_state & I_NEW);
		inode->i_state &= ~I_WILL_FREE;
 --->
	}

	inode->i_state |= I_FREEING;

It's normally covered by i_lock, but it's a problem if anyone looks at the
pair without taking i_lock.

Even flipping the order:

	if (!drop) {
		inode->i_state |= I_WILL_FREE;
		spin_unlock(&inode->i_lock);
		write_inode_now(inode, 1);
		spin_lock(&inode->i_lock);
		WARN_ON(inode->i_state & I_NEW);
		inode->i_state |= I_FREEING;
		inode->i_state &= ~I_WILL_FREE;
	} else {
		inode->i_state |= I_FREEING;
	}

isn't a guarantee of the order in which the compiler will do things AIUI.
Maybe I've been listening to Paul McKenney too much.  So the WRITE_ONCE()
should guarantee that both bits will change atomically.

Note that ocfs2_drop_inode() looks a tad suspicious:

	int ocfs2_drop_inode(struct inode *inode)
	{
		struct ocfs2_inode_info *oi = OCFS2_I(inode);

		trace_ocfs2_drop_inode((unsigned long long)oi->ip_blkno,
					inode->i_nlink, oi->ip_flags);

		assert_spin_locked(&inode->i_lock);
		inode->i_state |= I_WILL_FREE;
		spin_unlock(&inode->i_lock);
		write_inode_now(inode, 1);
		spin_lock(&inode->i_lock);
		WARN_ON(inode->i_state & I_NEW);
		inode->i_state &= ~I_WILL_FREE;

		return 1;
	}

David