linux-kernel - Re: [RFC PATCH] kernel/fork: fix CLONE_CHILD

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Message-ID: <20160819132511.GH32619@dhcp22.suse.cz>
Date:   Fri, 19 Aug 2016 15:25:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        William Preston <wpreston@...e.com>,
        Roland McGrath <roland@...k.frob.com>,
        Andreas Schwab <schwab@...e.com>
Subject: Re: [RFC PATCH] kernel/fork: fix CLONE_CHILD_CLEARTID regression in
 nscd

On Fri 12-08-16 11:41:13, Michal Hocko wrote:
> On Wed 03-08-16 23:08:04, Oleg Nesterov wrote:
> > sorry for delay, I am travelling till the end of the week.
> 
> Same here...
> 
> > On 08/01, Michal Hocko wrote:
[...]
> > > We should also check for vfork because
> > > this is killable since d68b46fe16ad ("vfork: make it killable").
> > 
> > Hmm, why? Can't understand... In any case this check doesn't look right, the
> > comment says "a killed vfork parent" while tsk->vfork_done != NULL means it
> > is a vforked child.
> > 
> > So if we want this change, why we can't simply do
> > 
> > 	-	if (!(tsk->flags & PF_SIGNALED) &&
> > 	+	if (!(tsk->signal->flags & SIGNAL_GROUP_COREDUMP) &&
> > 
> > ?
> 
> This is what I had initially. But then the comment above the check made
> me worried that the parent of vforked child might get confused if the
> flag is cleared. I might have completely misunderstood the point of the
> comment though. So if you believe that vfork_done check is incorrect I
> can drop it. It shouldn't have any effect on the nscd usecase AFAIU.

So should I drop the vfork check and repost or we do not care about this
"regression" and declare nscd broken because it relies on a behavior
which is not in fact guaranteed by the kernel?
-- 
Michal Hocko
SUSE Labs