linux-kernel - Re: [take25 1/6] kevent: Description.

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Message-Id: <200611232249.56886.hhh@imada.sdu.dk>
Date:	Thu, 23 Nov 2006 22:49:56 +0100
From:	Hans Henrik Happe <hhh@...da.sdu.dk>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...l.org>, netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chase Venters <chase.venters@...entec.com>,
	Johann Borck <johann.borck@...sedata.com>,
	linux-kernel@...r.kernel.org, Jeff Garzik <jeff@...zik.org>
Subject: Re: [take25 1/6] kevent: Description.

On Thursday 23 November 2006 21:00, Ulrich Drepper wrote:
> Evgeniy Polyakov wrote:
> > uidx is an index, starting from which there are unread entries. It is
> > updated by userspace when it commits entries, so it is 'consumer'
> > pointer, while kidx is an index where kernel will put new entries, i.e.
> > 'producer' index. We definitely need them both.
> > Userspace can only update (implicitly by calling kevent_commit()) uidx.
> 
> Right, which is why exporting this entry is not needed.  Keep the 
> interface as small as possible.
> 
> Userlevel has to maintain its own index.  Just assume kevent_wait 
> returns 10 new entries and you have multiple threads.  In this case all 
> threads take their turns and pick an entry from the ring buffer.  This 
> basically has to be done with something like this (I ignore wrap-arounds 
> here to simplify the example):
> 
>    int getidx() {
>      while (uidx < kidx)
>         if (atomic_cmpxchg(uidx, uidx + 1, uidx) == 0)
>           return uidx;
>      return -1;
>    }

I don't know if this falls under the simplification, but wouldn't there be a 
race when reading/copying the event data? I guess this could be solved with 
an extra user index. 

--

Hans Henrik Happe 
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