linux-kernel - Re: WARNING: syz-executor still has locks held!

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Message-ID: <20190320194512.GD8696@dhcp22.suse.cz>
Date:   Wed, 20 Mar 2019 20:45:12 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     syzbot <syzbot+b70f2aabc707c69c9239@...kaller.appspotmail.com>,
        chanho.min@....com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, pavel@....cz,
        rafael.j.wysocki@...el.com, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk, Ingo Molnar <mingo@...e.hu>
Subject: Re: WARNING: syz-executor still has locks held!

On Wed 20-03-19 18:30:42, Oleg Nesterov wrote:
> On 03/20, Michal Hocko wrote:
> >
> > [Cc Ingo and Chanho Min - the thread starts here
> > http://lkml.kernel.org/r/0000000000004cdec6058485b2ce@google.com]
> >
> > On Wed 20-03-19 16:00:54, Oleg Nesterov wrote:
> > > On 03/20, Michal Hocko wrote:
> > > >
> > > > On Wed 20-03-19 14:24:11, Oleg Nesterov wrote:
> > > > > On 03/20, Michal Hocko wrote:
> > > > > >
> > > > > > Yes we do hold the cgred mutex while calling freezable_schedule but why
> > > > > > are we getting a warning is not really clear to me. The task should be
> > > > > > hidden from the freezer so why do we warn at all?
> > > > >
> > > > > try_to_freeze() calls debug_check_no_locks_held() and this makes sense.
> > > >
> > > > Yes it does. But it already ignores PF_NOFREEZE tasks and I fail to see
> > > > why is PF_FREEZER_SKIP any different.
> > >
> > > But they differ. PF_NOFREEZE is a "sticky" flag for kthreads. Set by default,
> > > cleared by set_freezable() if you want a freezable kthread.
> > >
> > > PF_FREEZER_SKIP means that a sleeping freezable task will call try_to_freeze()
> > > right after schedule() returns, so try_to_freeze_tasks() can safely count it as
> > > "already frozen".
> >
> > But the fundamental semantic is the same right? Both might be sitting on
> > locks that might interfere with other tasks and we should be _extra_
> > careful when using them. In an ideal world, none of them is really
> > needed.
> 
> Ah, it seems that we misunderstood each other... see below.
> 
> > So my question remains. Can we drop the warning for PF_FREEZER_SKIP
> > tasks as well?
> 
> But why?

To drop the warning which led to the revert.

> It is obviously wrong to call try_to_freeze() with a lock held.

It is but the question what do we care about more. A task blocking
suspend so that the operation fails or a process being frozen with cgred
held. We have discussed that during the review of the original patch and
concluded that this is the only way for now AFAIR.

> Probably you meant the
> 
> 	if (!(current->flags & PF_NOFREEZE))
> 
> check in try_to_freeze() when you said "already ignores PF_NOFREEZE tasks".

Yes, that is the check I've had in mind.
-- 
Michal Hocko
SUSE Labs