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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 26, 2008 at 02:41:12PM +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.

Given that the only API call shared with normal completions is
complete() I'd rather make this a primitive of it's own, even if
internally implemented as completions.

Also please add kerneldoc comments for all new APIs eported to modules.