linux-kernel - Re: [PATCH 10/11] VFS: take a shared lock for create/remove directory operations.

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Message-ID: <20241223051941.GK1977892@ZenIV>
Date: Mon, 23 Dec 2024 05:19:41 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neilb@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/11] VFS: take a shared lock for create/remove
 directory operations.

On Fri, Dec 20, 2024 at 01:54:28PM +1100, NeilBrown wrote:

> Once the exclusive "update" lock is obtained on the dentry we must make
> sure it wasn't unlinked or renamed while we slept.  If it was we repeat
> the lookup.

> +		if (
> +			/* Was unlinked while we waited ?*/
> +			d_unhashed(dentry) ||
> +			/* Or was dentry renamed ?? */
> +			dentry->d_parent != base ||
> +			dentry->d_name.hash != last->hash ||
> +			!d_same_name(dentry, base, last)
> +		) {
> +			rcu_read_unlock();
> +			spin_unlock(&dentry->d_lock);
> +			lock_map_release(&dentry->d_update_map);
> +			dput(dentry);
> +			goto retry;
> +		}
> +		rcu_read_unlock();
> +	}
> +	dentry->d_flags |= DCACHE_PAR_UPDATE;
> +	spin_unlock(&dentry->d_lock);

... and now __d_unalias() moves it to new place, making all the checks
you've just done completely useless.