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

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Date:	Thu, 26 Jun 2008 13:33:25 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Dave Chinner <david@...morbit.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, 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

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