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Date:   Fri, 9 Oct 2020 11:58:11 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Naresh Kamboju <naresh.kamboju@linaro.org>
Cc:     "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, linux-api@vger.kernel.org,
        open list <linux-kernel@vger.kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Kees Cook <keescook@chromium.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Jens Axboe <axboe@kernel.dk>, Shuah Khan <shuah@kernel.org>,
        lkft-triage@lists.linaro.org
Subject: Re: selftests: pidfd: pidfd_wait hangs on linux next kernel on all
 devices
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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