linux-kernel - Re: [RFC 09/15] PM / Hibernate: user, implement user

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Date:	Thu, 25 Mar 2010 23:14:33 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Pavel Machek <pavel@....cz>, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Nigel Cunningham <ncunningham@...a.org.au>
Subject: Re: [RFC 09/15] PM / Hibernate: user, implement user_ops writer

On Wednesday 24 March 2010, Jiri Slaby wrote:
> On 03/24/2010 09:42 PM, Pavel Machek wrote:
> >> +	if (test_bit(TODO_CLOSED, to_do_flags))
> >> +		return -EIO;
> >> +
> >> +	to_do_buf = buf;
> >> +	wmb();
> >> +	set_bit(TODO_WORK, to_do_flags);
> >> +	wake_up_interruptible(&to_do_wait);
> > 
> > Uhuh, open-coded barriers... these need to be commented, and I guess
> > you just should not play this kind of trickery.
> 
> It's just to ensure the to_do_buf store is not reordered with the
> set_bit. I wanted to avoid locks as too heavy tools here.

No, please use them, at least in a prototype version.

We can always optimize things out later, but doing optimizations upfront
doesn't really work well from my experience.

So, if you'd use a lock somewhere, please use it, or maybe use a completion if
that fits the design better.  In the majority of cases it's not as heavy wieght
as it seems at first sight.

Rafael
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