From: Neil Brown <neilb@suse.de>
To: Jan Blunck <jblunck@suse.de>
Date: Mon, 13 Mar 2006 08:57:09 +1100
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Message-ID: <17428.39221.861124.797480@cse.unsw.edu.au>
Cc: Kirill Korotaev <dev@openvz.org>, akpm@osdl.org, viro@zeniv.linux.org.uk,
       olh@suse.de, bsingharora@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Fix shrink_dcache_parent() against shrink_dcache_memory() race (3rd updated patch)]
In-Reply-To: message from Jan Blunck on Friday March 10
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On Friday March 10, jblunck@suse.de wrote:
> On Fri, Mar 10, Neil Brown wrote:
> 
> > -static void prune_dcache(int count)
> > +static void prune_dcache(int count, struct super_block *sb)
> >  {
> >  	spin_lock(&dcache_lock);
> >  	for (; count ; count--) {
> > @@ -417,8 +425,10 @@ static void prune_dcache(int count)
> >   			spin_unlock(&dentry->d_lock);
> >  			continue;
> >  		}
> > -		/* If the dentry was recently referenced, don't free it. */
> > -		if (dentry->d_flags & DCACHE_REFERENCED) {
> > +		/* If the dentry was recently referenced, or is for
> > +		 * a unmounting filesystem, don't free it. */
> > +		if ((dentry->d_flags & DCACHE_REFERENCED) ||
> > +		    (dentry->d_sb != sb && dentry->d_sb->s_root == NULL)) {
> >  			dentry->d_flags &= ~DCACHE_REFERENCED;
> >   			list_add(&dentry->d_lru, &dentry_unused);
> >   			dentry_stat.nr_unused++;
> 
> You have to down_read the rw-semaphore sb->s_umount since sb->s_root is
> protected by it :(

No you don't.
sb->s_root is set precisely twice for any filesystem.
Once when the filesystem is mounted, typically in the fill_super
function, and once in generic_shutdown_super where it is set to NULL.

There is no need to lock against the first change as the sb is not
globally visible until after s_root is set.  So for the present
purpose we only need to worry about the second change.

For this, the current usage of dcache_lock is enough to remove any
possible race.  generic_shutdown_super sets s_root to NULL and then
takes dcache_lock (via wait_for_prunes) before it cares if anyone has
noticed the NULL or not.  prune_dcache holds dcache_lock while testing
for NULL.  So there is no room for a race.

s_umount is sometimes taken before testing s_root.  This is always
because the sb has just been found on the list of all superblocks, and
so the thread doesn't hold a proper reference to it.  In our case
we are holding a dentry which in turn holds a real reference to the
superblock.

So I stand by my patch - s_root can be safely tested here.

NeilBrown
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