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SubjectRe: [PATCH] mm: might_sleep warning
On Tue, 6 Mar 2018 16:04:06 -0500 Pavel Tatashin <pasha.tatashin@oracle.com> wrote:

> > > > > spin_lock(&deferred_zone_grow_lock);
> > > > > - static_branch_disable(&deferred_pages);
> > > > > + deferred_zone_grow = false;
> > > > > spin_unlock(&deferred_zone_grow_lock);
> > > > > + static_branch_disable(&deferred_pages);
> > > > >
> > > > > /* There will be num_node_state(N_MEMORY) threads */
> > > > > atomic_set(&pgdat_init_n_undone, num_node_state(N_MEMORY));
> > > >
> > > > Kinda ugly, but I can see the logic behind the decisions.
> > > >
> > > > Can we instead turn deferred_zone_grow_lock into a mutex?
> >
> > (top-posting repaired. Please don't top-post).
> >
> > > [CCed everyone]
> > >
> > > Hi Andrew,
> > >
> > > I afraid we cannot change this spinlock to mutex
> > > because deferred_grow_zone() might be called from an interrupt context if
> > > interrupt thread needs to allocate memory.
> > >
> >
> > OK. But if deferred_grow_zone() can be called from interrupt then
> > page_alloc_init_late() should be using spin_lock_irq(), shouldn't it?
> > I'm surprised that lockdep didn't detect that.
>
> No, page_alloc_init_late() cannot be called from interrupt, it is
> called straight from:
> kernel_init_freeable(). But, I believe deferred_grow_zone(): can be called:
>
> get_page_from_freelist()
> _deferred_grow_zone()
> deferred_grow_zone()

That's why page_alloc_init_late() needs spin_lock_irq(). If a CPU is
holding deferred_zone_grow_lock with enabled interrupts and an
interrupt comes in on that CPU and the CPU runs deferred_grow_zone() in
its interrupt handler, we deadlock.

lockdep knows about this bug and should have reported it.

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