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

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Date:	Fri, 24 Nov 2006 19:14:06 +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 08:06:59AM -0800, Ulrich Drepper (drepper@...hat.com) wrote:
> Evgeniy Polyakov wrote:
> >>I know this is how it's done now.  But it is not where it has to end. 
> >>IMO we have to get to a solution where new events are posted to the ring 
> >>buffer asynchronously, i.e., without a thread calling kevent_wait.  And 
> >>then you need the extra parameter and verification.  Even if it's today 
> >>not needed we have to future-proof the interface since it cannot be 
> >>changed once in use.
> >
> >There is a special flag in kevent_user to wake it if there are no ready
> >events - kernel thread which has added new events will set it and thus
> >subsequent kevent_wait() will return with updated indexes - userspace
> >must check indexes after kevent_wait().
> 
> You misunderstand.  I don't want to return without waiting unconditionally.
> 
> There is a race which has to be closed.  It's exactly the same as in the 
> futex syscall.  I've shown the interaction between the kernel and the 
> thread in the previous mail.  There is inevitably a time difference 
> between the thread checking whether the ring buffer is empty and the 
> kernel putting the thread to sleep in the kevent_wait call.
> 
> This is no problem with the current kevent_wait implementation since the 
> ring buffer is not filled asynchronously.  But if/when it will be the 
> kernel might add something to the ring buffer _after_ the thread checks 
> for an empty ring buffer and _before_ it enters the kernel in the 
> kevent_wait syscall.
> 
> The kevent_wait syscall will only wake the thread when a new event is 
> posted.  We do not in general want it to be woken when the ring buffer 
> is non empty.  This would create far too many unnecessary wakeups it 
> there is more than one thread working on the queue.
> 
> With the addition parameters for kevent_wait indicating when the calling 
> thread last checked the ring buffer the kernel can find out whether the 
> decision to call kevent_wait was made based on outdated information or 
> not.  Outdated in the case a new event has been posted.  In this case 
> the thread is not put to sleep but instead returns.

Read my mail again.

If kernel has put data asynchronously it will setup special flag, thus 
kevent_wait() will not sleep and will return, so thread will check new
entries and process them.

> -- 
> ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, 
> CA ❖

-- 
	Evgeniy Polyakov
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