Re: selftests: pidfd: pidfd_wait hangs on linux next kernel on all devices

[Christian Brauner <christian.brauner@ubuntu.com>]

On Fri, Oct 02, 2020 at 11:01:34PM +0530, Naresh Kamboju wrote:
> On Thu, 24 Sep 2020 at 21:51, Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> >
> > On Thu, Sep 24, 2020 at 04:33:17PM +0200, Christian Brauner wrote:
> > > On Wed, Sep 23, 2020 at 07:52:05PM +0530, Naresh Kamboju wrote:
> > > > selftests: pidfd: pidfd_wait hangs on linux next kernel on x86_64,
> > > > i386 and arm64 Juno-r2
> > > > These devices are using NFS mounted rootfs.
> > > > I have tested pidfd testcases independently and all test PASS.
> > > >
> > > > The Hang or exit from test run noticed when run by run_kselftest.sh
> > > >
> > > > pidfd_wait.c:208:wait_nonblock:Expected sys_waitid(P_PIDFD, pidfd,
> > > > &info, WSTOPPED, NULL) (-1) == 0 (0)
> > > > wait_nonblock: Test terminated by assertion
> > > >
> > > > metadata:
> > > >   git branch: master
> > > >   git repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> > > >   git commit: e64997027d5f171148687e58b78c8b3c869a6158
> > > >   git describe: next-20200922
> > > >   make_kernelversion: 5.9.0-rc6
> > > >   kernel-config:
> > > > http://snapshots.linaro.org/openembedded/lkft/lkft/sumo/intel-core2-32/lkft/linux-next/865/config
> > > >
> > > > Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> > >
> > > Thanks for reproting this. I'm taking a look now!
> >
> > Ok, this is a simple race in the selftests, that I overlooked and which
> > is more likely to hit when there's a lot of processes running on the
> > system. Basically the child process hasn't SIGSTOPed itself yet but the
> > parent is already calling waitid() on a O_NONBLOCK pidfd. Since it
> > doesn't find a WSTOPPED process it returns -EAGAIN correctly.
> >
> > The fix for this is to move the line where we're removing the O_NONBLOCK
> > property from the fd before the waitid() WSTOPPED call so we hang until
> > the child becomes stopped.
> >
> > So I believe the fix is:
> 
> I have tested this patch and the pidfd_wait test did not hang this time.
> 
> Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> 
> test log link
> https://lkft.validation.linaro.org/scheduler/job/1813223

Thanks for testing, Naresh. I've applied the patch:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/commit/?h=pidfd_o_nonblock&id=01361b665a26ef0c087e53f14cf310d1cfe0cf98

Christian