This is how we fixed it for now. But it requires paths locking starting from the root. Otherwise they can become parent-child in a moment after sucessfull check.
On Thu, Sep 24, 2015 at 11:28 AM, Sergi Vladykin <sergi.vlady...@gmail.com> wrote: > May be just check that they are not parent-child within the tx? > > Sergi > Igniters, > > We revealed concurrency problem in IGFS and I would like to discuss > possible solutions to it. > > Consider the following file system structure: > root > |-- A > | |-- B > | | |-- C > | |-- D > > ... two concurrent operations in different threads: > T1: move(/A/B, /A/D); > T2: move(/A/D, /A/B/C); > > ... and how IGFS handles it now: > T1: verify that "/A/B" and "/A/D" exist, they are not child-parent to each > other, etc. -> OK. > T2: do the same for "A/D" and "A/B/C" -> OK. > T1: get IDs of "/A", "/A/B" and "/A/D" to lock them later inside tx. > T2: get IDs of "/A", "/A/D", "/A/B" and "/A/B/C" to lock them later inside > tx. > > T1: Start pessimistic tx, lock IDs of "/A", "/A/B", "/A/D", perform move -> > OK. > root > |-- A > | |-- D > | | |-- B > | | | |-- C > > T2: Start pessimistic tx, lock IDs of "/A", "/A/D", "/A/B" and > "/A/B/C" (*directory > structure already changed at this time!*), perform move -> OK. > root > |-- A > B > |-- D > | |-- C > | | |-- B (loop!) > > File system is corrupted. Folders B, C and D are not reacheable from root. > > To fix this now we additionaly check if directory structure is still > valid *inside > transaction*. It works, no more corruptions. But it requres taking locks on > the whole paths *including root*. So move, delete and mkdirs opeartions > *can > no longer be concurrent*. > > Probably there is a way to relax this while still ensuring consistency, but > I do not see how. One idea is to store real path inside each entry. This > way we will be able to ensure that it is still at a valid location without > blocking parents, so concurrnecy will be restored. But we will have to > propagate strucutral changes to children. E.g. move of a folder with 100 > items will lead to update of >100 cache entries. Not so good. > > Any other ideas? > > Vladimir. >
