Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 16 Feb 2018 10:03:15 -0800 | Subject | Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry_kill() |
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On Fri, Feb 16, 2018 at 7:09 AM, John Ogness <john.ogness@linutronix.de> wrote: > dentry_kill() holds dentry->d_lock and needs to acquire both > dentry->d_inode->i_lock and dentry->d_parent->d_lock. This cannot be > done with spin_lock() operations because it's the reverse of the > regular lock order. To avoid ABBA deadlocks it is done with two > trylock loops. > > Trylock loops are problematic in two scenarios:
I don't mind this patch series per se (although I would really like Al to ack it), but this particular patch I hate.
Why?
> Avoid the trylock loops by using dentry_lock_inode() and lock_parent() > which take the locks in the appropriate order. As both functions drop > dentry->lock briefly, this requires rechecking of the dentry content > as it might have changed after dropping the lock.
I think the trylock should be done first, and then you don't need that recheck for the common case.
I realize that the recheck itself isn't expensive, but it's mostly about the code flow and the comment:
> + * Recheck refcount as it might have been incremented while > + * d_lock was dropped.
the thing is, 99.9% of the time the d_lock wasn't dropped, so that "while d_lock was dropped" comment is misleading.
Re-organizing it to do the trylock fastpath explicitly here and not bothering with the re-check etc crid for the common case is the rioght thing to do.
And the old code was actually organized exactly that way, with a
if (inode && unlikely(!spin_trylock(&inode->i_lock))) goto failed;
at the top.
But instead of having that unlikely "failed" case do the complex thing, you made the *normal* case do the complex thing.
So NAK on this.
It should be fairly trivial to fix, and make the "failed" thing do it right.
Linus
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