linux-ext4 - Re: possible deadlock in start_this

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Message-ID: <YCU9OR7SfRpwl4+4@dhcp22.suse.cz>
Date:   Thu, 11 Feb 2021 15:20:41 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Theodore Ts'o <tytso@....edu>, Linux-MM <linux-mm@...ck.org>
Subject: Re: possible deadlock in start_this_handle (2)

On Thu 11-02-21 13:25:33, Matthew Wilcox wrote:
> On Thu, Feb 11, 2021 at 02:07:03PM +0100, Michal Hocko wrote:
> > On Thu 11-02-21 12:57:17, Matthew Wilcox wrote:
> > > > current->flags should be always manipulated from the user context. But
> > > > who knows maybe there is a bug and some interrupt handler is calling it.
> > > > This should be easy to catch no?
> > > 
> > > Why would it matter if it were?
> > 
> > I was thinking about a clobbered state because updates to ->flags are
> > not atomic because this shouldn't ever be updated concurrently. So maybe
> > a racing interrupt could corrupt the flags state?
> 
> I don't think that's possible.  Same-CPU races between interrupt and
> process context are simpler because the CPU always observes its own writes
> in order and the interrupt handler completes "between" two instructions.

I have to confess I haven't really thought the scenario through. My idea
was to simply add a simple check for an irq context into ->flags setting
routine because this should never be done in the first place. Not only
for scope gfp flags but any other PF_ flags IIRC.

-- 
Michal Hocko
SUSE Labs