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

From: Al Viro
Date: Tue May 24 2016 - 19:41:41 EST

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@xxxxxxxxxxxx> wrote:
> >
> > On 05/24/2016 02:25 PM, Matthew McClintock wrote:
> >> On May 24, 2016, at 2:16 PM, Larry Finger <Larry.Finger@xxxxxxxxxxxx> wrote:
> >>>
> >>> On 05/24/2016 02:13 PM, Matthew McClintock wrote:
> >>>> Iâm 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? Iâm 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.