Ian Kent <raven@xxxxxxxxxx> writes: > On Sat, 2016-09-17 at 22:10 +0200, Mateusz Guzik wrote: >> On Wed, Sep 14, 2016 at 02:14:45PM +0800, Ian Kent wrote: >> > If an automount mount is clone(2)ed into a file system that is >> > propagation private, when it later expires in the originating >> > namespace subsequent calls to autofs ->d_automount() for that >> > dentry in the original namespace will return ELOOP until the >> > mount is manually umounted in the cloned namespace. >> > >> > In the same way, if an autofs mount is triggered by automount(8) >> > running within a container the dentry will be seen as mounted in >> > the root init namespace and calls to ->d_automount() in that namespace >> > will return ELOOP until the mount is umounted within the container. >> > >> > Also, have_submounts() can return an incorect result when a mount >> > exists in a namespace other than the one being checked. >> > >> > @@ -460,7 +460,7 @@ static int autofs4_d_manage(struct dentry *dentry, bool >> > rcu_walk) >> > >> > if (ino->flags & AUTOFS_INF_WANT_EXPIRE) >> > return 0; >> > - if (d_mountpoint(dentry)) >> > + if (is_local_mountpoint(dentry)) >> > return 0; >> > inode = d_inode_rcu(dentry); >> > if (inode && S_ISLNK(inode->i_mode)) >> >> This change is within RCU lookup. >> >> is_local_mountpoint may end up calling __is_local_mountpoint, which will >> optionally take the namespace_sem lock, resulting in a splat: > > Yes, that's a serious problem I missed. > > snip ... > >> I don't know this code. Perhaps it will be perfectly fine performance wise to >> just drop out of RCU lookup in this case. > > It's a bit worse than that. > > I think being able to continue the rcu-walk for an already mounted dentry that > is not being expired is an important part of the performance improvement given > by the series that added this. > > Can you confirm that Neil? > > But for the case here the existing test can allow rcu-walk to continue for a > dentry that would attempt to trigger an automount so it's also a bug in the > existing code. I don't think the existing code is buggy. As I read __follow_mount_rcu if DCACHE_NEED_AUTOMOUNT is set on the dentry after return from d_manage the code will break out of the rcu walk. > Any thoughts on how we can handle this Neil, I'm having a bit of trouble working > out how to resolve this one. I believe in this case d_mountpoint is enough for the rcu walk case. If the mountpoint turns out not to be local __follow_mount_rcu will kick out of the rcu walk as it will return false. Because: return !mounted && !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT); I am not quite certain about the non-rcu case. That can't be is_local_mountpoint as that is inside a spinlock and is_local_mountpoint can sleep. Perhaps d_mountpoint is simply correct for autofs4_d_manage. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
