linux-kernel - Re: [-next regression] TCP window full with EPOLLWAKEUP

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Message-Id: <201205202034.24541.rjw@sisk.pl>
Date:	Sun, 20 May 2012 20:34:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	NeilBrown <neilb@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [-next regression] TCP window full with EPOLLWAKEUP

On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
> On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
> > On Sunday, May 20, 2012, Jiri Slaby wrote:
> > > Hi,
> > > 
> > > a bisection shows that with the following commit from -next:
> > > commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
> > > Author: Arve Hjønnevåg <arve@...roid.com>
> > > Date:   Tue May 1 21:33:34 2012 +0200
> > > 
> > >     epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll
> > > events are ready
> > > 
> > > ====
> > > 
> > > one of mono programs I use stops receiving data from the network.
> > > Wireshark shows that the TCP window of a connection is filled. This
> > > means the program does not read the data fast enough after requesting
> > > the data.
> > > 
> > > If I revert that commit on the top of -next (20120518), everything works
> > > as expected.
> > 
> > Hmm.  I suppose that the failing program doesn't set EPOLLWAKEUP by mistake,
> > does it?
> 
> If it doesn't, we can assume that epi-ws is always NULL and all of the added
> overhead comes from the function calls.  So, I wonder if the appended patch
> makes any difference?

Having thought more about this I have to say this doesn't seem to make much
sense, because in that case you'd see some progress, although probably a bit
slower than before.

So, I think what happens is that the application tries to set EPOLLWAKEUP,
but doesn't have the capability, so the entire operation fails for it, but
it doesn't check the return value.

I wonder if the following helps, then.

Thanks,
Rafael


---
 fs/eventpoll.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux/fs/eventpoll.c
===================================================================
--- linux.orig/fs/eventpoll.c
+++ linux/fs/eventpoll.c
@@ -1711,7 +1711,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
 
 	/* Check if EPOLLWAKEUP is allowed */
 	if ((epds.events & EPOLLWAKEUP) && !capable(CAP_EPOLLWAKEUP))
-		goto error_tgt_fput;
+		epds.events &= ~EPOLLWAKEUP;
 
 	/*
 	 * We have to check that the file structure underneath the file descriptor
--
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