Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Al Viro
Date: Wed Jun 12 2013 - 20:21:20 EST

On Wed, Jun 12, 2013 at 05:01:19PM -0700, Linus Torvalds wrote:
> I'd actually suggest we do *not* remove any existing d_lock usage
> outside of the particular special cases we want to optimize, which at
> least from Davidlohr's profile is just dput() (which has shown up a
> lot before) and dget_parent() (which I'm not sure why it happens so
> much on his load, but it really seems trivially safe to optimistically
> do under just the RCU lock).

Actually, dget_parent() change might be broken; the thing is, the assumptions
are more subtle than "zero -> non-zero only happens under ->d_lock". It's
actually "new references are grabbed by somebody who's either already holding
one on the same dentry _or_ holding ->d_lock". That's what d_invalidate()
check for ->d_count needs for correctness - caller holds one reference, so
comparing ->d_count with 2 under ->d_lock means checking that there's no other
holders _and_ there won't be any new ones appearing.

Consider the following situation:
X is dentry of a/b
Y is dentry of a/b/c
Z is dentry of d/e

A holds a reference to Y and enters dget_parent(Y)
B holds a reference to X and enters d_invalidate(X)
A picks the value of Y->d_parent (== X)
C moves Y to Z
B grabs ->d_lock on X
B checks X->d_count; it's 1, we deduce that no other references exist or
are going to appear
A does atomic_inc_not_zero(&X->d_count). And since it's not zero (it's 1,
actually), we've just grabbed an extra reference on X that was not going
to appear according to B...

> That said, I do wonder if we could do something like
> "atomic_inc_not_zero()" on the d_count, and only if it is zero (which
> won't be horribly unusual, since for leaf dentries that nobody else is
> using) we'd do the whole locking sequence.

Same correctness issue as above, I'm afraid...

> End result: I think it would be interesting to try this all out, and
> it could be a noticeable win under some cases, but it *definitely*
> needs a lot of timing and testing to see which ways it goes..


What's more, we need the underlying assumptions documented very clearly for
any such change; it's _not_ as simple as "protect transitions from zero to
non-zero and we are done" ;-/
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