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SubjectRe: [RFC 09/15] PM / Hibernate: user, implement user_ops writer
On Fri 2010-03-26 10:34:59, Jiri Slaby wrote:
> On 03/25/2010 11:14 PM, Rafael J. Wysocki wrote:
> > 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.
>
> That's it, I don't think a lock is appropriate here (I didn't even think
> of that) -- I don't know what to lock (OK, I see it, but it's not that
> clear). There is no potential for race per se, I only need to disable
> reordering (which locks do as a side-effect). I need the steps to be
> done in the A-B order where there is a barrier appropriate. Here, A is
> store to to_do_buf, B is set_bit. It's I set to_do_buf, flag that it may
> be used, the consumer will see the flag and use to_do_buf, in this
> order.

Could you just use same locking user.c uses? It does not really do any
magic...
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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