linux-kernel - Re: [PATCH 14/19] VFS: Ensure no async updates happening in directory being removed.

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Message-ID: <20250208220653.GQ1977892@ZenIV>
Date: Sat, 8 Feb 2025 22:06:53 +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>,
	Jeff Layton <jlayton@...nel.org>,
	Dave Chinner <david@...morbit.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/19] VFS: Ensure no async updates happening in
 directory being removed.

On Fri, Feb 07, 2025 at 09:06:58PM +0000, Al Viro wrote:
> On Thu, Feb 06, 2025 at 04:42:51PM +1100, NeilBrown wrote:
> > vfs_rmdir takes an exclusive lock on the target directory to ensure
> > nothing new is created in it while the rmdir progresses.  With the
> > possibility of async updates continuing after the inode lock is dropped
> > we now need extra protection.
> > 
> > Any async updates will have DCACHE_PAR_UPDATE set on the dentry.  We
> > simply wait for that flag to be cleared on all children.
> 
> > +static void d_update_wait(struct dentry *dentry, unsigned int subclass)
> > +{
> > +	/* Note this may only ever be called in a context where we have
> > +	 * a lock preventing this dentry from becoming locked, possibly
> > +	 * an update lock on the parent dentry.  The must be a smp_mb()
> > +	 * after that lock is taken and before this is called so that
> > +	 * the following test is safe. d_update_lock() provides that
> > +	 * barrier.
> > +	 */
> > +	if (!(dentry->d_flags & DCACHE_PAR_UPDATE))
> > +		return
> > +	lock_acquire_exclusive(&dentry->d_update_map, subclass,
> > +			       0, NULL, _THIS_IP_);
> 
> What the fuck?
> 
> > +	spin_lock(&dentry->d_lock);
> > +	wait_var_event_spinlock(&dentry->d_flags,
> > +				!check_dentry_locked(dentry),
> > +				&dentry->d_lock);
> > +	spin_unlock(&dentry->d_lock);
> > +	lock_map_release(&dentry->d_update_map);
> > +}
> 
> OK, I realize that it compiles, but it should've raised all
> kinds of red flags for anyone reading that.  return + <newline> is
> already fishy, but having the next line indented *less* than that
> return is firmly in the "somebody's trying to hide something nasty
> here" territory, even without parsing the damn thing.

Incidentally, that's where lockdep warnings you've mentioned are
coming from...