Re: [PATCH 1/6] Extend completions to provide XFS object flush requirements

Daniel Walker
2008-06-26T20:32:42Z

On Thu, 2008-06-26 at 14:41 +1000, Dave Chinner wrote:
> XFS object flushing doesn't quite match existing completion semantics. It
> mixed exclusive access with completion. That is, we need to mark an object as
> being flushed before flushing it to disk, and then block any other attempt to
> flush it until the completion occurs.
>
> To do this we introduce:
>
> void init_completion_flush(struct completion *x)
> which initialises x->done = 1
>
> void completion_flush_start(struct completion *x)
> which blocks if done == 0, otherwise decrements done to zero and
> allows the caller to continue.
>
> bool completion_flush_start_nowait(struct completion *x)
> returns a failure status if done == 0, otherwise decrements done
> to zero and returns a "flush started" status. This is provided
> to allow flushing to begin safely while holding object locks in
> inverted order.
>
> This replaces the use of semaphores for providing this exclusion
> and completion mechanism.

I think there is some basis to make the changes that you have here.
Specifically this email and thread,

http://lkml.org/lkml/2008/4/15/232

However, I don't like how your implementing this as specifically a
"flush" mechanism for XFS, and the count is limited to just 1 .. There
are several other places that do this kind of counting with semaphores,
and have counts above 1..

> +
> +static inline void completion_flush_start(struct completion *x)
> +{
> + wait_for_completion(x);
> +}

Above seems completely pointless.. I would just call
wait_for_completion(), and make the rest of the interface generic.

Daniel