linux-kernel - Re: [syzbot] [mm?] KCSAN: data-race in mas_wr_store_entry / mtree_range

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Message-ID: <CANpmjNMJWLq2H-Wdrx5SjPjc566V4siqod2Gy6-Dms2sn2+8PQ@mail.gmail.com>
Date: Mon, 23 Sep 2024 13:57:55 +0200
From: Marco Elver <elver@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: syzbot <syzbot+e01fa33e67abb0b3b3bb@...kaller.appspotmail.com>, 
	Liam.Howlett@...cle.com, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	syzkaller-bugs@...glegroups.com, vbabka@...e.cz
Subject: Re: [syzbot] [mm?] KCSAN: data-race in mas_wr_store_entry / mtree_range_walk

On Mon, 23 Sept 2024 at 11:44, 'Lorenzo Stoakes' via syzkaller-bugs
<syzkaller-bugs@...glegroups.com> wrote:
>
> On Mon, Sep 23, 2024 at 02:04:23AM GMT, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    88264981f208 Merge tag 'sched_ext-for-6.12' of git://git.k..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1237ec27980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=e6702f5f2b8ed242
> > dashboard link: https://syzkaller.appspot.com/bug?extid=e01fa33e67abb0b3b3bb
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
>
> Thanks for the report, investigating.
>
> > Unfortunately, I don't have any reproducer for this issue yet.
>
> I suspect given this is so timing-specific, a reproducer might be difficult.

FWIW, syzbot currently does not try to find reproducers for KCSAN,
because it's too non-deterministic.

The best strategy would be for a developer who has an intuition for
what might be going on to create a stress-test to reproduce.

> This suggests we are failing to acquire an RCU lock on mmap() (though we
> have the write mmap lock).
>
> Maybe we missed an RCU lock at some point, but I'm a little baffled as to
> what could have changed in recent series to adjust this.
>
> I will dig into this and see what's going on.

Thanks!