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

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Message-ID: <20061123115240.GA20294@2ka.mipt.ru>
Date:	Thu, 23 Nov 2006 14:52:41 +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 Wed, Nov 22, 2006 at 03:46:42PM -0800, Ulrich Drepper (drepper@...hat.com) wrote:
> Evgeniy Polyakov wrote:
> >+ int kevent_wait(int ctl_fd, unsigned int num, __u64 timeout);
> >+
> >+ctl_fd - file descriptor referring to the kevent queue 
> >+num - number of processed kevents 
> >+timeout - this timeout specifies number of nanoseconds to wait until 
> >there is +		free space in kevent queue 
> >+
> >+Return value:
> >+ number of events copied into ring buffer or negative error value.
> 
> This is not quite sufficient.  What we also need is a parameter which 
> specifies which ring buffer the code assumes is currently active.  This 
> is just like the EWOULDBLOCK error in the futex.  I.e., the kernel 
> doesn't move the thread on the wait list if the index has changed. 
> Otherwise asynchronous ring buffer filling is impossible.  Assume this
> 
>     thread                             kernel
> 
>     get current ring buffer idx
> 
>     front and tail pointer the same
> 
>                                        add new entry to ring buffer
> 
>                                        bump front pointer
> 
>     call kevent_wait()
> 
> 
> With the interface above this leads to a deadlock.  The kernel delivered 
> the event and is done with it.

Kernel does not put there a new entry, it is only done inside
kevent_wait(). Entries are put into queue (in any context), where they can be obtained
from only kevent_wait() or kevent_get_events().

-- 
	Evgeniy Polyakov
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