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

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Date:	Thu, 21 May 2015 11:44:14 +0000 (GMT)
From:	Maninder Singh <maninder1.s@...sung.com>
To:	Ingo Molnar <mingo@...nel.org>
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

EP-F6AA0618C49C4AEDA73BFF1B39950BAB

Hi Ingo,

>> >> 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?

we sent this fix because we were doing static analysis of kernel code by tool coverity analyzer (Prevent), and it shows this unlock mismatch,
Thats why we think to report this whether it is right or not .

>Your patch is breaking the kernel, and badly so.

Sorry for the noise .

>Thanks,

>	Ingo


Thanks