Re: [PATCH RFC ticketlock] Auto-queued ticketlock

[Raymond Jennings <shentino@gmail.com>]

On Wed, 2013-06-12 at 13:26 -0700, Linus Torvalds wrote:
> On Wed, Jun 12, 2013 at 1:03 PM, Davidlohr Bueso <davidlohr.bueso@hp.com> wrote:
> >
> > According to him:
> >
> > "the short workload calls security functions like getpwnam(),
> > getpwuid(), getgrgid() a couple of times. These functions open
> > the /etc/passwd or /etc/group files, read their content and close the
> > files.
> 
> Ahh, ok. So yeah, it's multiple threads all hitting the same file

If that's the case and it's a bunch of reads, shouldn't they act
concurrently anyway?

I mean it's not like dentries are being changed or added or removed in
this case.

> I guess that /etc/passwd case is historically interesting, but I'm not
> sure we really want to care too deeply..
> 
> > I did a quick attempt at this (patch attached).
> 
> Yeah, that's wrong, although it probably approximates the dget() case
> (but incorrectly).
> 
> One of the points behind using an atomic d_count is that then dput() should do
> 
>    if (!atomic_dec_and_lock(&dentry->d_count, &dentry->d_count))
>       return;
> 
> at the very top of the function. It can avoid taking the lock entirely
> if the count doesn't go down to zero, which would be a common case if
> you have lots of users opening the same file. While still protecting
> d_count from ever going to zero while the lock is held.
> 
> Your
> 
> +       if (atomic_read(&dentry->d_count) > 1) {
> +               atomic_dec(&dentry->d_count);
> +               return;
> +       }
> +       spin_lock(&dentry->d_lock);
> 
> pattern is fundamentally racy, but it's what "atomic_dec_and_lock()"
> should do race-free.
> 
> For similar reasons, I think you need to still maintain the d_lock in
> d_prune_aliases etc. That's a slow-path, so the fact that we add an
> atomic sequence there doesn't much matter.
> 
> However, one optimization missing from your patch is obvious in the
> profile. "dget_parent()" also needs to be optimized - you still have
> that as 99% of the spin-lock case. I think we could do something like
> 
>    rcu_read_lock();
>    parent = ACCESS_ONCE(dentry->d_parent);
>    if (atomic_inc_nonzero(&parent->d_count))
>       return parent;
>    .. get d_lock and do it the slow way ...
>    rcu_read_unlock();
> 
> to locklessly get the parent pointer. We know "parent" isn't going
> away (dentries are rcu-free'd and we hold the rcu read lock), and I
> think that we can optimistically take *any* parent dentry that
> happened to be valid at one point. As long as the refcount didn't go
> down to zero. Al?
> 
> With dput and dget_parent() both being lockless for the common case,
> you might get rid of the d_lock contention entirely for that load. I
> dunno. And I should really think more about that dget_parent() thing a
> bit more, but I cannot imagine how it could not be right (because even
> with the current d_lock model, the lock is gotten *within*
> dget_parent(), so the caller can never know if it gets a new or an old
> parent, so there is no higher-level serialization going on - and we
> might as well return *either* the new or the old as such).
> 
> I really want Al to double-check me if we decide to try going down
> this hole. But the above two fixes to your patch should at least
> approximate the d_lock changes, even if I'd have to look more closely
> at the other details of your patch..
> 
>                 Linus
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