linux-ext4 - Re: possible deadlock in start_this

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Date:   Thu, 11 Feb 2021 14:26:30 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...e.com>
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, Feb 11, 2021 at 03:20:41PM +0100, Michal Hocko wrote:
> 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.

That's not automatically clear to me.  There are plenty of places
where an interrupt borrows the context of the task that it happens to
have interrupted.  Specifically, interrupts should be using GFP_ATOMIC
anyway, so this doesn't really make a lot of sense, but I don't think
it's necessarily wrong for an interrupt to call a function that says
"Definitely don't make GFP_FS allocations between these two points".