On Wed, Oct 01, 2014 at 01:16:15AM +0100, Al Viro wrote: > On Mon, Sep 29, 2014 at 11:42:18AM -0700, Paul E. McKenney wrote: > > > Assuming that incrementing the external name's reference count is > > atomic_add_unless, I could believe this part. Or if you have some > > locking that makes it impossible to increment the reference count > > in any case where there is any risk that it might be decremented > > to zero, I guess. > > > > Which might well be the pair of write_seqcount_begin() calls in __d_move(), > > now that I look more carefully. So if the name has to be in use somewhere > > before it can be copied, then a copy can only be created if there is at > > least one copy that is not currently being removed? If so, OK. > > Huh? copy_name() does copy a _reference_, not the name itself. All the > copying involved is source->d_name.name = target->d_name.name. And those > are simply unsigned char *. > > write_seqcount_begin() is irrelevant here. Look: all callers of > __d_move(x, y) are holding references both to x and y. Contributing to > the refcount of dentries themselves, that is, not the names. > > That gives exclusion between __d_move() and free_dentry() - the latter cannot > be called until dentry refcount reaches zero. RCU is completely irrelevant > here. In fact, no call chain leads to __d_move() under rcu_read_lock(). > You must hold the target dentry hard, or it could simply be freed right > under you. > > And __d_move() is taking ->d_lock on all dentries involved (in > addition to rename_lock serializing it system-wide). > > What could possibly lead to refcount zero being observed on target of > __d_move()? The history of any dentry is this: > * it is created by __d_alloc(). Nobody can see it until __d_alloc() > returns. Dentry refcount (not to be confused with refcount of external > name) is 1. > * it passes through some (usually - zero) __d_move() calls. > Some - as the first argument, some - as the second one. All those > calls are serialized by global seqlock - callers must hold rename_lock. > And all of them are done by somebody who is holding a counting reference > to dentries in question. > * counting references to dentry might be taken and dropped; > eventually refcount reaches zero (under ->d_lock) and no further > counting references can be taken after that. See __dentry_kill() - the > first thing it does is poisoning the refcount, so that any future > attempt to increment it would fail. __dentry_kill() (still under ->d_lock > of dentry, ->d_lock of its parent and ->i_lock of its inode) removes > dentry from the tree, from hash and from the alias list of inode; > Then it drops the locks. At that point the only search structure dentry > might be found in is shrink list; if it's not on such list, free_dentry() > is called immediately, otherwise it's marked so that the code processing > the shrink list in question would, as soon as it gets to that sucker, > remove it from the shrink list and call the same free_dentry(). And that's > the only thing done to such dentry by somebody finding it via a shrink list. > * once free_dentry() has been reached, dentry can can be only seen > by RCU lookups, and after the grace period ends it gets physically freed. > > free_dentry() isn't allowed to overlap __d_move(); to have that happen is > a serious dentry refcounting bug. No __d_move() is allowed _after_ > free_dentry() has been entered, either. Again, it would take a refcounting > bug for dentries to have that happen - basically, double dput() somewhere. > If that happens, all bets are off, of course - if dentry gets unexpectedly > freed under somebody who has grabbed a reference to it and has not dropped > it yet, we are fucked. > > Nothing outside of __d_move() is allowed to change ->d_name.name. RCU-critical > code is allowed to fetch and dereference it, and such code relies upon > a) freeing of name seen by somebody who'd done rcu_read_lock() being > delayed until after the matching rcu_read_unlock() > b) store of terminating NUL done by __d_alloc() (and never overwritten > afterwards) being seen by RCU-critical code that has found the pointer to > that name in dentry->d_name.name > > All other code accessing ->d_name.name is required to hold one of the locks > that are held by __d_move() and its callers. Grabbing any of those leads > to smp_mb() on alpha, which serves as data dependency barrier there, so > we don't need explicit barrier there as we do in RCU-critical places - guarding > NUL will be seen. Please accept my apologies for the noise! Thanx, Paul -- 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
