Re: [EDT][PATCH] kernel/exit.c : Fix missing read_unlock

Ingo Molnar
2015-05-21T10:58:28Z

* 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
> >>
> Subject: [PATCH 1/1] kernel/exit.c : Fix missing read_unlock
>
> >> This patch adds missing read_unlock if do_wait_thread or ptrace_do_wait
> >> returns non zero.
>
> Reported By Prevent Under Missing unlock category(program hangs):-
> missing_unlock: returning without unlocking tasklist_lock
> >>
> >> 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 and the changelog is lacking.
>
> >So the last time that code was touched upstream was 7 years ago:
>
> > commit 64a16caf5e3417ee32f670debcb5857b02a9e08e
> > Author: Oleg Nesterov <oleg@redhat.com>
> > Date: Wed Jun 17 16:27:40 2009 -0700
>
> > do_wait: simplify retval/tsk_result/notask_error mess
>
> >please explain whether what you fix is:
>
> > 1) an ancient bug that somehow nobody ever triggered (plus analysis
> > of why it wasn't triggered)
>
> > 2) a new bug introduced by commit XYZ (plus analysis)
>
> > 3) something else
>
> This issue is reported by Prevent Under category Missing Unlock, So
> we think it should be reported to maintainers.

Huh? In what way does your reply answer my questions?

Your patch is breaking the kernel, and badly so.

Thanks,

Ingo