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    SubjectRe: possible deadlock in shmem_mfill_atomic_pte
    On Mon, 13 Apr 2020, Yang Shi wrote:
    > On Tue, Mar 31, 2020 at 10:21 AM syzbot
    > <syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com> wrote:
    > >
    > > Hello,
    > >
    > > syzbot found the following crash on:
    > >
    > > HEAD commit: 527630fb Merge tag 'clk-fixes-for-linus' of git://git.kern..
    > > git tree: upstream
    > > console output: https://syzkaller.appspot.com/x/log.txt?x=1214875be00000
    > > kernel config: https://syzkaller.appspot.com/x/.config?x=27392dd2975fd692
    > > dashboard link: https://syzkaller.appspot.com/bug?extid=e27980339d305f2dbfd9
    > > compiler: gcc (GCC) 9.0.0 20181231 (experimental)
    > >
    > > Unfortunately, I don't have any reproducer for this crash yet.
    > >
    > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
    > > Reported-by: syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com
    > >
    > > WARNING: possible irq lock inversion dependency detected
    > > 5.6.0-rc7-syzkaller #0 Not tainted
    > > --------------------------------------------------------
    > > syz-executor.0/10317 just changed the state of lock:
    > > ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
    > > ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
    > > but this lock was taken by another, SOFTIRQ-safe lock in the past:
    > > (&(&xa->xa_lock)->rlock#5){..-.}
    > >
    > >
    > > and interrupts could create inverse lock ordering between them.
    > >
    > >
    > > other info that might help us debug this:
    > > Possible interrupt unsafe locking scenario:
    > >
    > > CPU0 CPU1
    > > ---- ----
    > > lock(&(&info->lock)->rlock);
    > > local_irq_disable();
    > > lock(&(&xa->xa_lock)->rlock#5);
    > > lock(&(&info->lock)->rlock);
    > > <Interrupt>
    > > lock(&(&xa->xa_lock)->rlock#5);
    > >
    > > *** DEADLOCK ***
    >
    > This looks possible. shmem_mfill_atomic_pte() acquires info->lock with
    > irq enabled.
    >
    > The below patch should be able to fix it:

    I agree, thank you: please send to akpm with your signoff and

    Reported-by: syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com
    Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
    Acked-by: Hugh Dickins <hughd@google.com>

    I bet that 4.11 commit was being worked on before 4.8 reversed the
    ordering of info->lock and tree_lock, changing spin_lock(&info->lock)s
    to spin_lock_irq*(&info->lock)s - this one is the only hold-out; and
    not using userfaultfd, I wouldn't have seen the lockdep report.

    >
    > diff --git a/mm/shmem.c b/mm/shmem.c
    > index d722eb8..762da6a 100644
    > --- a/mm/shmem.c
    > +++ b/mm/shmem.c
    > @@ -2399,11 +2399,11 @@ static int shmem_mfill_atomic_pte(struct
    > mm_struct *dst_mm,
    >
    > lru_cache_add_anon(page);
    >
    > - spin_lock(&info->lock);
    > + spin_lock_irq(&info->lock);
    > info->alloced++;
    > inode->i_blocks += BLOCKS_PER_PAGE;
    > shmem_recalc_inode(inode);
    > - spin_unlock(&info->lock);
    > + spin_unlock_irq(&info->lock);
    >
    > inc_mm_counter(dst_mm, mm_counter_file(page));
    > page_add_file_rmap(page, false);

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