On Fri, Feb 23, 2018 at 12:50:22AM +0100, John Ogness wrote:
> The trylock loop can be avoided with functionality similar to
> lock_parent(). The fast path tries the trylock first, which is likely
> to succeed. In the contended case it attempts locking in the correct
> order. This requires to drop dentry->d_lock first, which allows
> another task to free d_inode.
Wait a minute. _What_ allows another task to free ->d_inode on
a dentry we are holding a reference to? Any place like that is
a serious bug - after all, what's to prevent the same place
doing that to dentry of an opened file, with obvious ugly
results.
That's the whole reason why d_delete() is *NOT* making dentry
negative when refcount is greater than 1 (i.e. when somebody
else is holding a reference).
Rules for ->d_inode:
* initially NULL.
* only changes under ->d_lock
* __dentry_kill() makes it NULL after dentry has been
+ marked dead
+ evicted from all lists except possibly shrink one.
with ->d_lock held through all of that. The only thing
that can be done by anybody else with the ones stuck on
shrink list is actually freeing them.
Note that once __dentry_kill() is called, that's it - dentry
is ours, for all practical purposes. There'd better be no
other references to that sucker and we make sure that no new
ones will arise.
* prior to the call of __dentry_kill() any would-be changer
of ->d_inode must be holding a reference to dentry.
* changes from non-NULL to NULL are possible only when there's
nobody else holding references.
Changes from NULL to non-NULL _are_ possible (caller must be
holding a reference, but that's it). However, feeding a negative
dentry to your dentry_lock_inode() is an instant oops - it won't
live to the point where you would recheck ->d_inode for changes.
So if you see any place where positive could be changed to negative
under us, we do have a problem. Big one.
Refcount can change once we drop ->d_lock, but it can't get to zero -
our reference is still with us.
Note that ->d_parent *CAN* change, no matter how many references are
held. That's what rcu games in lock_parent() are about - dentry
can be moved and ex-parent could've been freed if that was the last
reference.
