linux-kernel - Re: [PATCH 04/10] AXFS: axfs

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Date:	Thu, 21 Aug 2008 17:12:26 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Jared Hulbert" <jaredeh@...il.com>
Cc:	Linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	"Jörn Engel" <joern@...fs.org>,
	tim.bird@...sony.com, cotte@...ibm.com, nickpiggin@...oo.com.au
Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c

On Thursday 21 August 2008, Jared Hulbert wrote:
> > Have you seen any benefit of the rwsem over a simple mutex? I would guess
> > that you can never even get into the situation where you get concurrent
> > readers since I haven't found a single down_read() in your code, only
> > downgrade_write()
> 
> We implemented a rwsem here because you can get concurrent readers.
> My understanding is that downgrade_write() puts the rewem into the
> same state as down_read().  Am I mistaken?

Your interpretation of downgrade_write is correct, but if every thread
always does

down_write();
serialized_code();
downgrade_write();
parallel_code();
up_read();

Then you still won't have any concurrency, because each thread trying
to down_write() will be blocked until the previous one has done its up_read(),
causing parallel_code() to be serialized as well.

In addition to that, I'd still consider it better to use a simple mutex
if parallel_code() is a much faster operation than serialized_code(), as it
is in your case, where only the memcpy is parallel and that is much slower
than the deflate.

	Arnd <><
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