Messages in this thread | | | Date | Sat, 12 Jun 2021 01:45:50 +0000 | From | Al Viro <> | Subject | Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates |
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On Wed, Jun 09, 2021 at 04:51:22PM +0800, Ian Kent wrote: > The inode operations .permission() and .getattr() use the kernfs node > write lock but all that's needed is to keep the rb tree stable while > updating the inode attributes as well as protecting the update itself > against concurrent changes.
Huh? Where does it access the rbtree at all? Confused...
> diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c > index 3b01e9e61f14e..6728ecd81eb37 100644 > --- a/fs/kernfs/inode.c > +++ b/fs/kernfs/inode.c > @@ -172,6 +172,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode) > { > struct kernfs_iattrs *attrs = kn->iattr; > > + spin_lock(&inode->i_lock); > inode->i_mode = kn->mode; > if (attrs) > /* > @@ -182,6 +183,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode) > > if (kernfs_type(kn) == KERNFS_DIR) > set_nlink(inode, kn->dir.subdirs + 2); > + spin_unlock(&inode->i_lock); > }
Even more so - just what are you serializing here? That code synchronizes inode metadata with those in kernfs_node. Suppose you've got two threads doing ->permission(); the first one gets through kernfs_refresh_inode() and goes into generic_permission(). No locks are held, so kernfs_refresh_inode() from another thread can run in parallel with generic_permission().
If that's not a problem, why two kernfs_refresh_inode() done in parallel would be a problem?
Thread 1: permission done refresh, all locks released now Thread 2: change metadata in kernfs_node Thread 2: permission goes into refresh, copying metadata into inode Thread 1: generic_permission() No locks in common between the last two operations, so we generic_permission() might see partially updated metadata. Either we don't give a fuck (in which case I don't understand what purpose does that ->i_lock serve) *or* we need the exclusion to cover a wider area.
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