linux-kernel - Re: [EDT][PATCH] kernel/exit.c : Fix missing read

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Date:	Thu, 21 May 2015 12:58:20 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Maninder Singh <maninder1.s@...sung.com>
Cc:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"mhocko@...e.cz" <mhocko@...e.cz>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"ionut.m.alexa@...il.com" <ionut.m.alexa@...il.com>,
	"peter@...leysoftware.com" <peter@...leysoftware.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vaneet Narang <v.narang@...sung.com>,
	AKHILESH KUMAR <akhilesh.k@...sung.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [EDT][PATCH] kernel/exit.c : Fix missing read_unlock


* Maninder Singh <maninder1.s@...sung.com> wrote:

> EP-F6AA0618C49C4AEDA73BFF1B39950BAB
> >> Hi,
> >> 
> >> From: Maninder Singh <maninder1.s@...sung.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@...sung.com>
> >> Signed-off-by: Vaneet Narang <v.narang@...sung.com>
> >> Reviewd-by: Akhilesh Kumar <akhilesh.k@...sung.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@...hat.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
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