Re: [patch] inotify.

Nick Piggin
2005-06-17T01:30:42Z

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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