On Mon, Aug 04, 2008 at 11:03:27AM +1000, Neil Brown wrote: > The locking bothers me. > The VFS seems to have a general policy of doing the locking itself to > make life easier for filesystem implementors (or to make it harder for > them to mess up, depending on your perspective). > > The current issue seems to suggest that the locking provided by the > VFS is no longer adequate, so each filesystem is needing to create > something itself. That suggests to me a weakness in the model. > Possibly the VFS should give up trying to be in control and let > filesystems do their own locking. Possibly there are still things > that the VFS can do which are universally good. I think these are > questions that should be addressed. > Maybe they have already been addressed and I missed the conversation > (that wouldn't surprise me much). But seeing words like "hack" > suggests to me that it hasn't. So I want to make sure I understand > the problem properly and deeply before giving my blessing to a hack. > > So: what are the issues? > > Obviously readdir can race with create and you don't want them > tripping each other up. The current VFS approach to this is i_mutex. > Any place which modifies a directory or does a lookup in a directory > takes i_mutex to ensure that the directory doesn't change. > > This is probably fairly limiting. With a tree-structured directory > you only really need to lock the 'current node' of the tree. > So any access would lock the top node, find which child to follow, > then lock the child and unlock the parent. Rebalancing might need to > be creative as you cannot lock a parent while holding a lock on the > child, but that isn't insurmountable. > So I could argue that holding i_mutex across a lookup or create or > readdir maybe isn't ideal. It would be interesting to explore the > possibility of dropping i_mutex across calls into the filesystem. > By the time the filesystem is called, we really only need to be > locking the names (dentries) rather than the whole directory. > More on this later... > > Some filesystems want to restructure directories and times that are > independent of any particular access. This might be defragmentation > or rebalancing or whatever. Clearly there needs to be mutual > exclusion between this and other accesses such as readdir and lookup. > The VFS clearly cannot help with this. It doesn't know when > rebalancing might happen or are what sort of granularity. So the > filesystem must do it's own locking. > It strikes me that this sort of locking would best be done by > spinlocks at the block level rather than a per-inode mutex. However > I haven't actually implemented this (yet) so I cannot be certain. > > This is what is causing the current problem (for JFFS2 at least). > JFF2 has a per-inode lock which protects against internally visible > changes to the inode. Further (and this is key) this lock is held > across the filldir callback. > i_mutex is also held across the filldir callback, but there is an > important difference. It is taken by the VFS, not the filesystem, > and it is guaranteed always to be held across the filldir callback. > So the filldir callback can call e.g. lookup without further locking. > > Backing up a little: given that the filesystem implementor chose to > use per-inode locking to protect internal restructuring (which is > certainly an easier option), why not just use i_mutex? The reason > is that a create operation might trigger system-wide garbage > collection which could trigger restructuring of the current inode, > which would lead to an A-A deadlock (as the create is waiting for the > garbage collection, and so still holding i_mutex). > > So, given that background, it is possible to see some more possible > solutions (other than the ones already mentioned). > > - drop the internal lock across filldir. > It could be seen a impolite to hold any locks across a callback > that are not documented as being held. > This would put an extra burden on the filesystem, but it shouldn't > be a particularly big burden. > A filesystem needs to be able to find the 'next' entry from a > sequential 'seek' pointer so that is the most state that needs to > be preserved. It might be convenient to be able to keep more state > (pointers into data structures etc). All this can be achieved with > fairly standard practice: > a/ have a 'version' number per inode which is updated when any > internal restructure happens. > b/ when calling filldir, record the version number, drop the lock > call filldir, reclaim the lock, check the version number > c/ if the version number matches (as it mostly will) just keep > going. If it doesn't jump back to the top of readdir where > we decode the current seek address. > > Some filesystems have problems implementing seekdir/telldir so they > might not be totally happy here. I have little sympathy for such > filesystems and feel the burden should be on them to make it work. > > - use i_mutex to protect internal changes too, and drop i_mutex while > doing internal restructuring. This would need some VFS changes so > that dropping i_mutex would be safe. It would require some way to > lock an individual dentry. Probably you would lock it under > i_mutex by setting a flag bit, wait for the flag on some inode-wide > waitqueue, and drop the lock by clearing the flag and waking the > waitqueue. And you are never allowed to look at ->d_inode if the > lock flag is set. Isn't there a lot of kernel code that assumes that following ->d_inode is safe? Or are you only worried about looking at certain filesystem-internal fields of d_inode? The locking required to keep the filesystem namespace consistent is difficult and important, so I think changing it would require an extremely careful description of the new design and a commitment from some filesystem developers to actually take advantage of it.... --b. > Of these I really like the second. Refining the i_mutex down to a > per-name lock before calling in to the filesystem seems like a really > good idea and should be good for scalability and large directories. > It isn't something to be done lightly though. The filesystem would > still be given i_mutex held, but would be allowed to drop it if it > wanted to. We could have an FS_DROPS_I_MUTEX similar to the current > FS_RENAME_DOES_D_MOVE. > > For the first, I really like the idea that a lock should not be held > across calls the filldir. I feel that a filesystem doing that is > "wrong" in much the same way that some think that recursing into the > filesystem as nfsd does is "wrong". In reality neither is "wrong", we > just need to work together and negotiate and work out the best way to > meet all of our needs. > > So what should we do now? I think that for JFFS2 to drop and reclaim > f->sem in readdir would be all of 20 lines of code, including updating > a ->version counter elsewhere in the code. Replicating that in all > the filesystems that need it would probably be more code than the nfsd > change though. > > On the other hand, if we implement the nfsd change, it will almost > certainly never go away, even if all filesystems eventually find that > they don't need it any more because someone improves the locking > rules in the VFS. Where as the code in the filesystems could quite > possibly go away when they are revised to make better use of the > locking regime. So I don't think that is an ideal situation either. > > If I had time, I would work on modifying the VFS to allow filesystems > to drop i_mutex. However I don't have time at the moment, so I'll > leave the decision to be made by someone else (Hi Bruce! I'll > support whatever you decide). > > But I think it is important to understand what is really going on and > not just implement a hack that works around the problem. I think I do > understand now, so I am a lot happier. 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