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

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Message-ID: <20061124121312.GD32545@2ka.mipt.ru>
Date:	Fri, 24 Nov 2006 15:13:12 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	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 Fri, Nov 24, 2006 at 03:05:31PM +0300, Evgeniy Polyakov (johnpol@....mipt.ru) wrote:
> On Thu, Nov 23, 2006 at 02:33:16PM -0800, Ulrich Drepper (drepper@...hat.com) wrote:
> > Evgeniy Polyakov wrote:
> > >+ int kevent_commit(int ctl_fd, unsigned int start, 
> > >+ 	unsigned int num, unsigned int over);
> > 
> > I think we can simplify this interface:
> > 
> >    int kevent_commit(int ctl_fd, unsigned int new_tail,
> >                      unsigned int over);
> > 
> > The kernel sets the ring_uidx value to the 'new_tail' value if the tail 
> > pointer would be incremented (module wrap around) and is not higher then 
> > the current front pointer.  The test will be a bit complicated but not 
> > more so than what the current code has to do to check for mistakes.
> > 
> > This approach has the advantage that the commit calls don't have to be 
> > synchronized.  If one thread sets the tail pointer to, say, 10 and 
> > another to 12, then it does not matter whether the first thread is 
> > delayed.  If it will eventually be executed the result is simply a no-op 
> > and since second thread's action supersedes it.
> > 
> > Maybe the current form is even impossible to use with explicit locking 
> > at userlevel.  What if one thread, which is about to call kevent_commit, 
> > if indefinitely delayed.  Then this commit request's value is never 
> > taken into account and the tail pointer is always short of what it 
> > should be.
> 
> I like this interface, although current one does not allow special

...does not require...

> synchronization in userspace, since it calculates if new commit is in
> the area where previous commit was.
> Will change for the next release.

-- 
	Evgeniy Polyakov
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