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Date: Thu, 21 May 2015 12:56:22 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Frans Klaver <fransklaver@gmail.com>
Cc: Maninder Singh <maninder1.s@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>, oleg@redhat.com,
        Michal Hocko <mhocko@suse.cz>, Peter Zijlstra <peterz@infradead.org>,
        Rik van Riel <riel@redhat.com>, ionut.m.alexa@gmail.com,
        Peter Hurley <peter@hurleysoftware.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "v.narang@samsung.com" <v.narang@samsung.com>,
        AKHILESH KUMAR <akhilesh.k@samsung.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [EDT][PATCH] kernel/exit.c : Fix missing read_unlock
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* Frans Klaver <fransklaver@gmail.com> wrote:

> On Thu, May 21, 2015 at 8:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Maninder Singh <maninder1.s@samsung.com> wrote:
> >
> >>  EP-F6AA0618C49C4AEDA73BFF1B39950BAB
> >> Hi,
> >>
> >> From: Maninder Singh <maninder1.s@samsung.com>
> >>
> >> Subject: [PATCH 1/1] kernel/exit.c : Fix missing task_unlock
> >>
> >> This patch adds missing read_unlock if do_wait_thread or ptrace_do_wait
> >> returns non zero.
> >>
> >> Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
> >> Signed-off-by: Vaneet Narang <v.narang@samsung.com>
> >> Reviewd-by: Akhilesh Kumar <akhilesh.k@samsung.com>
> >> ---
> >>  kernel/exit.c |    8 ++++++--
> >>  1 files changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/exit.c b/kernel/exit.c
> >> index 22fcc05..31a061f 100644
> >> --- a/kernel/exit.c
> >> +++ b/kernel/exit.c
> >> @@ -1486,12 +1486,16 @@ repeat:
> >>       tsk = current;
> >>       do {
> >>               retval = do_wait_thread(wo, tsk);
> >> -             if (retval)
> >> +             if (retval) {
> >> +                     read_unlock(&tasklist_lock);
> >>                       goto end;
> >> +             }
> >>
> >>               retval = ptrace_do_wait(wo, tsk);
> >> -             if (retval)
> >> +             if (retval) {
> >> +                     read_unlock(&tasklist_lock);
> >>                       goto end;
> >> +             }
> >>
> >>               if (wo->wo_flags & __WNOTHREAD)
> >>                       break;
> >
> > That's surprising <snip>
> 
> Still it looks like it is a legitimate change. I don't see where the 
> unlock would be done otherwise.

No, it does not look like a legitimate change, that's why I asked the 
questions. I think this patch breaks the kernel badly.

As it is explained in the comments as well, the various wait-loop 
functions (do_wait_thread(), ptrace_do_wait()) fundamentally unlock 
the tasklist_lock if they return an error.

NAK.

Thanks,

	Ingo
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