Re: possible deadlock in shmem_uncharge

[Hugh Dickins]

On Wed, 15 Apr 2020, Yang Shi wrote:
> On Wed, Apr 15, 2020 at 7:04 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > On Mon, 13 Apr 2020, Yang Shi wrote:
> > >
> > > It looks shmem_uncharge() is just called by __split_huge_page() and
> > > collapse_file(). The collapse_file() has acquired xa_lock with irq
> > > disabled before acquiring info->lock, so it is safe.
> > > __split_huge_page() is called with holding xa_lock with irq enabled,
> > > but lru_lock is acquired with irq disabled before acquiring xa_lock.
> > >
> > > So, it is unnecessary to acquire info->lock with irq disabled in
> > > shmem_uncharge(). Can syzbot try the below patch?
> >
> > But I disagree with the patch below.  You're right that IRQ-disabling
> > here is unnecessary, given its two callers; but I'm not sure that we
> > want it to look different from shmem_charge() and all other info->lock
> > takers; and, more importantly, I don't see how removing the redundant
> > IRQ-saving below could make it any less liable to deadlock.
> 
> Yes, I realized the patch can't suppress the lockdep splat. But,
> actually I didn't understand how this deadlock could happen because
> info_lock is acquired with IRQ disabled before acquiring
> user_shm_lock. So, interrupt can't come in at all if I didn't miss
> anything.

I think the story it's trying to tell is this (but, like most of us,
I do find Mr Lockdep embarrassingly difficult to understand; and I'm
not much good at drawing race diagrams either):

CPU0 was in user_shm_unlock(), it's got shmlock_user_lock, then an
interrupt comes in. It's an endio kind of interrupt, which goes off
to test_clear_page_writeback(), which wants the xa_lock on i_pages.

Meanwhile, CPU1 was doing some SysV SHM locking, it's got as far as
shmem_lock(), it has acquired info->lock, and goes off to user_shm_lock()
which wants shmlock_user_lock.

But sadly, CPU2 is splitting a shmem THP, calling shmem_uncharge()
that wants info->lock while outer level holds xa_lock on i_pages:
with interrupts properly disabled, but that doesn't help.

Now, that story doesn't quite hold up as a deadlock, because shmem
doesn't use writeback tags; and (unless you set shmem_enabled "force")
I don't think there's a way to get shmem THPs in SysV SHM (and are
they hole-punchable? maybe through MADV_REMOVE); so it looks like
we're talking about different inodes.

But lockdep is right to report it, and more thought might arrive at
a more convincing scenario.  Anyway, easily fixed and best fixed.

(But now I think my patch must wait until tomorrow.)

Hugh