On Fri, 2019-09-27 at 15:09 +0800, Ian Kent wrote: > On Fri, 2019-09-27 at 09:35 +0800, Ian Kent wrote: > > On Thu, 2019-09-26 at 14:52 -0500, Goldwyn Rodrigues wrote: > > > An access to automounted filesystem can deadlock if it is a bind > > > mount on shared mounts. A user program should not deadlock the > > > kernel > > > while automount waits for propagation of the mount. This is > > > explained > > > at https://bugzilla.redhat.com/show_bug.cgi?id=1358887#c10 > > > I am not sure completely blocking automount is the best solution, > > > so please reply with what is the best course of action to do > > > in such a situation. > > > > > > Propagation of dentry with DCACHE_NEED_AUTOMOUNT can lead to > > > propagation of mount points without automount maps and not under > > > automount control. So, do not propagate them. > > > > Yes, I'm not sure my comments about mount propagation in that > > bug are accurate. > > > > This behaviour has crept into the kernel in reasonably recent > > times, maybe it's a bug or maybe mount propagation has been > > "fixed", not sure. > > > > I think I'll need to come up with a more detailed description > > of what is being done for Al to be able to offer advice. > > > > I'll get to that a bit later. > > To duplicate this problem use an autofs indirect map > that uses bind mounts and has offsets: > > test / :/exports \ > /tmp :/exports/tmp \ > /lib :/exports/lib > > and add: > > /bind /etc/auto.exports > > to /etc/auto.master. > > Finally create the bind mount directories: > > mkdir -p /exports/lib /exports/tmp > > Then, on a broken kernel, eg. 4.13.9-300.fc27: > > ls /bind/test > > will result in: > > /etc/auto.exports on /bind type autofs > (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,indirec > t,pipe_ino=45485) > /dev/mapper/fedora_f27-root on /bind/test type ext4 > (rw,relatime,seclabel,data=ordered) > /etc/auto.exports on /bind/test/lib type autofs > (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=45485) > /etc/auto.exports on /exports/lib type autofs > (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=45485) > /etc/auto.exports on /bind/test/tmp type autofs > (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=45485) > /etc/auto.exports on /exports/tmp type autofs > (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=45485) > > these mount entries, not all of which have been mounted by autofs. > > Whereas on a kernel that isn't broken, eg. 4.11.8-300.fc26, the same > ls command will result in: > > /etc/auto.exports on /bind type autofs > (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,indirec > t,pipe_ino=42067) > /etc/auto.exports on /bind/test/lib type autofs > (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=42067) > /etc/auto.exports on /bind/test/tmp type autofs > (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,offset, > pipe_ino=42067) > > these mount entries, all of which have been mounted by autofs (and > are what's needed for these offset mount constructs). > > If the /bind mount is made propagation slave or private at mount > by automount the problem doesn't happen and that is the workaround > I implemented in autofs. Actually that's not quite right, there should be a bind mount at /bind/test as well but it's not present. Although I expect this will happen with a rootless multi-mount as well, aka. test \ /tmp :/exports/tmp \ /lib :/exports/lib Leave it with me while I investigate further. > > I initially thought this was the result of a "fix" in the mount > propagation code but it occurred to me that propagation is meant > to occur between mount trees not within them so this might be a > bug. > > I probably should have worked out exactly what upstream kernel > this started happening in and then done a bisect and tried to > work out if the change was doing what it was supposed to. > > Anyway, I'll need to do that now for us to discuss this sensibly. > > > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx> > > > > > > diff --git a/fs/pnode.c b/fs/pnode.c > > > index 49f6d7ff2139..b960805d7954 100644 > > > --- a/fs/pnode.c > > > +++ b/fs/pnode.c > > > @@ -292,6 +292,9 @@ int propagate_mnt(struct mount *dest_mnt, > > > struct > > > mountpoint *dest_mp, > > > struct mount *m, *n; > > > int ret = 0; > > > > > > + if (source_mnt->mnt_mountpoint->d_flags & > > > DCACHE_NEED_AUTOMOUNT) > > > + return 0; > > > + > > > > Possible problem with this is it will probably prevent mount > > propagation in both directions which will break stuff. > > > > I had originally assumed the problem was mount propagation > > back to the parent mount but now I'm not sure that this is > > actually what is meant to happen. > > > > > /* > > > * we don't want to bother passing tons of arguments to > > > * propagate_one(); everything is serialized by namespace_sem, > > >
