On Wed, Nov 28, 2018 at 08:08:06PM +0000, Will Deacon wrote: > I spent some more time looking at this today... > > On Fri, Nov 23, 2018 at 06:05:25PM +0000, Will Deacon wrote: > > Doing some more debugging, it looks like the usual failure case is where > > one CPU clears the inode field in the dentry via: > > > > devpts_pty_kill() > > -> d_delete() // dentry->d_lockref.count == 1 > > -> dentry_unlink_inode() > > > > whilst another CPU gets a pointer to the dentry via: > > > > sys_getdents64() > > -> iterate_dir() > > -> dcache_readdir() > > -> next_positive() > > > > and explodes on the subsequent inode dereference when trying to pass the > > inode number to dir_emit(): > > > > if (!dir_emit(..., d_inode(next)->i_ino, ...)) > > > > Indeed, the hack below triggers a warning, indicating that the inode > > is being cleared concurrently. > > > > I can't work out whether the getdents64() path should hold a refcount > > to stop d_delete() in its tracks, or whether devpts_pty_kill() shouldn't > > be calling d_delete() like this at all. > > So the issue is that opening /dev/pts/ptmx creates a new pty in /dev/pts, > which disappears when you close /dev/pts/ptmx. Consequently, when we tear > down the dentry for the magic new file, we have to take the i_node rwsem of > the *parent* so that concurrent path walkers don't trip over it whilst its > being freed. I wrote a simple concurrent program to getdents(/dev/pts/) in > one thread, whilst another opens and closes /dev/pts/ptmx: it crashes the > kernel in seconds. I also made a testcase and verified that your fix is fine. I also tried replacing open-close on /dev/ptmx with mkdir-rmdir but that does not trigger the error. > Patch below, but I'd still like somebody else to look at this, please. I wonder why no inode_lock on parent is needed for devpts_pty_new(), but I'm obviously not a VFS expert... So your patch looks good to me and clearly solves the issue. thanks, Jan > Will > > --->8 > > diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c > index c53814539070..50ddb95ff84c 100644 > --- a/fs/devpts/inode.c > +++ b/fs/devpts/inode.c > @@ -619,11 +619,17 @@ void *devpts_get_priv(struct dentry *dentry) > */ > void devpts_pty_kill(struct dentry *dentry) > { > - WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC); > + struct super_block *sb = dentry->d_sb; > + struct dentry *parent = sb->s_root; > > + WARN_ON_ONCE(sb->s_magic != DEVPTS_SUPER_MAGIC); > + > + inode_lock(parent->d_inode); > dentry->d_fsdata = NULL; > drop_nlink(dentry->d_inode); > d_delete(dentry); > + inode_unlock(parent->d_inode); > + > dput(dentry); /* d_alloc_name() in devpts_pty_new() */ > }