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

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Date:	Fri, 27 Jun 2008 11:52:41 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	xfs@....sgi.com, matthew@....cx, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] Extend completions to provide XFS object flush
	requirements

On Thu, Jun 26, 2008 at 01:33:25PM -0700, Daniel Walker wrote:
> 
> 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..

Agreed - but the extension has to start somewhere. So, do I simply
add a "init_completion_count()" that passes a count value for the
completion (i.e. replaces init_completion_flush())?

> > +
> > +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.

Except then wait_for_completion_nowait() makes absolutely no sense ;)
If i use wait_for_completion() for this, then perhaps the
non-blocking version becomes "try_wait_for_completion()". Would
this be acceptible?

i.e. the extra functions in the completion API would be:

	void init_completion_count(struct completion *x, int count);
	int try_wait_for_completion(struct completion *x);
	int completion_in_progress(struct completion *x);

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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