linux-kernel - Re: Unexpected cascaded epoll behavior

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Date:	Mon, 26 Jan 2009 12:36:04 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Pavel Pisa <pisa@....felk.cvut.cz>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Unexpected cascaded epoll behavior - my mistake or kernel bug

On Mon, 26 Jan 2009, Davide Libenzi wrote:

> On Mon, 26 Jan 2009, Pavel Pisa wrote:
> 
> > Hello Davide,
> > 
> > thanks for fast reply and effort.
> > 
> > I have tested your patch with patched and unpatched 2.6.28.2 kernel.
> > The outputs are attached. The patched kernel passes all your tests.
> > 
> > But I have bad news. My library code does not fall into busy loop
> > after your patching but my FIFO tests do not work even in
> > single level epoll scenario. The strace output is attached as well.
> > I try to do more testing tomorrow. I have returned from weekend trip
> > at the evening today and I have not much time for deeper analysis.
> > 
> > But it looks like write level sensitive events are not triggering
> > for epoll at all. The pipe is not fill by any character and specified
> > timeout is triggered with next message as fail results.
> >   @ pipe #X evptrig wr timeout
> >   @ pipe #X evptrig rd timeout
> >  
> > If you want to test the code on your box, download library
> > 
> > http://cmp.felk.cvut.cz/~pisa/ulan/ul_evpoll-090123.tar.gz
> > 
> > The simple "make" in the top directory should work.
> 
> It'd be really great if you could spare me having to dig into few 
> thousands lines of userspace library code, and you could isolate a little 
> bit better what is the expected result, and the error result.

Never mind, I looked myself and I'm able to replicate it with the simple 
attached test program. Looking into it ...



- Davide


View attachment "epoll_pipe_pingpong.c" of type "TEXT/x-csrc" (2757 bytes)