linux-kernel - Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c

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Message-ID: <20160524234133.GA14480@ZenIV.linux.org.uk>
Date:	Wed, 25 May 2016 00:41:33 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Matthew McClintock <mmcclint@...eaurora.org>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c

On Tue, May 24, 2016 at 05:31:51PM -0500, Matthew McClintock wrote:
> 
> > On May 24, 2016, at 2:36 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
> > 
> > On 05/24/2016 02:25 PM, Matthew McClintock wrote:
> >> On May 24, 2016, at 2:16 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
> >>> 
> >>> On 05/24/2016 02:13 PM, Matthew McClintock wrote:
> >>>> I’m seeing this too, same commit if you want another person to test/reproduce.
> >>> 
> >>> If you do a pull today, does that fix your problem?
> >> 
> >> Hmm, no. Which commit am I looking for? I’m on a56f489502e28caac56c8a0735549740f0ae0711
> > 
> > Commit 84787c572d402644dca4874aba73324d9f8e3948 is working for me. I have a fixup in lib/iov_iter.c with a dump_stack() call if the fixup was needed. That dump is not triggered. I do not seem to have a56f489502e yet.
> 
> Still seeing the issue on top of tree and the above commit. Re-ran bisection just to be sure.

Guys, the bug is real and definitely still there.
	char c;
	struct iovec v[2] = {{&c, 0}, {&c, 1}};
	readv(0, v, 2);
will trigger it just fine with stdin on e.g. tty.  It needs fixing and I'll
post a fix as soon as it gets through the local testing.  In the meanwhile,
I would like to know what in userland is doing that kind of call - kernel
certainly shouldn't end up in an infinite loop on that, but it's bloody odd
and I wonder what's going on in userland code to result in that call.

Again, I understand what's going on kernel-side; the only tricky part is how
to fix it without bringing the nasal daemons back.  I think I have a solution
and I'm going to post it tonight if it survives the local beating.  In any
case, the testcase above deserves being added to LTP - it's a real regression.