Re: [patch] inotify.

[Nick Piggin <nickpiggin@yahoo.com.au>]

Robert Love wrote:
> On Thu, 2005-06-16 at 10:52 -0700, Zach Brown wrote:

> > > +       if (likely(!atomic_read(&inode->inotify_watch_count)))
> > > +               return;
> >
> > Are there still platforms that implement atomic_read() with locks?  I
> > wonder if there isn't a cheaper way for inodes to find out that they're
> > not involved in inotify.. maybe an inode function pointer that is only
> > set to queue_event when watchers are around?
>
>
> I don't know what esoteric architectures are doing, but the solution
> needs to be atomic (or we need to say "we don't care about races"--but
> its hard not to care about a pointer race).  On x86, at least, an
> atomic_read() is trivial.
>
> I actually would not mind being racey (in a safe way) or finding a
> cheaper solution, especially if we could remove
> inode->inotify_watch_count altogether (and not replace it with
> anything).
>
> But the overhead here is not biting us (we just went through some
> off-list benchmarking that led to the inclusion of this check, in fact).

What we could do is just check list_empty(&inode->inotify_watchers)
and remove the atomic count completely.

We don't actually care about getting an exact count at all, just
whether or not it is empty, and in that case using list_empty is
no more racy than checking an atomic count, both are done outside
any locks.

It is basically just a lock avoidance heuristic. But I think count
is superfluous - off with its head!

--
SUSE Labs, Novell Inc.

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