On 12 July 2011 13:45, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > Michal Suchanek <hramrach@xxxxxxxxxx> writes: > >> On 12 July 2011 10:30, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: >>> Michal Suchanek <hramrach@xxxxxxxxxx> writes: >>> >>>> The locking order is likely determined by the structure of the union >>>> and not some system-wide order of filesystems so assuming the readonly >>>> layers are locked as well you will probably get a deadlock with >>>> technically correct mount: >>>> >>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/upper /tmpoverlay >>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/tmpoverlay /overlay >>>> >>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/upper2 /tmpoverlay2 >>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/tmpoverlay2 /overlay2 >>>> >>>> because now lower1 and lower2 are differently ordered in the two >>>> overlays. >>> >>> Overlayfs never locks both upper and lower at the same time, which means >>> there's no AB-BA locking dependency. The lock orderings are: >>> >>> -> /overlay >>> -> /lower1 >>> -> /tmpoverlay >>> -> /lower2 >>> -> /upper >>> -> /overlay2 >>> -> /lower2 >>> -> /tmpoverlay2 >>> -> /lower1 >>> -> /upper2 >>> >>> As you can see there's no nesting of lower2 and lower1 into each other. >>> >>> When you combine two filesystems, a completely new ordering is created >>> each time, there's no possibility to make an AB-BA nesting. At least I >>> cannot see one. >> >> Except you can get in situation where overlay locks lower1 and >> tmpoverlay waits for lower2 > > Note: tmpoverlay lock does *not* nest into lower1 lock, they are both on > the same nesting level. There's no dependency between the two. > >> which is held by overlay2 waiting for >> lower1 in tmpoverlay2. > > No deadlock there. > That's nice. You can still do mount --bind /lower1 /lower2/lower1 mount --bind /lower2 /lower1/lower2 Which is technically not against usage guidelines, unlike mount --bind /upper /lower1/upper If crossing mount boundaries is forbidden try with symlinks or hardlinks. Thanks Michal -- 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
