Re: [PATCH v2] mm: workingset: replace IRQ-off check with a lockdep assert.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2019-02-11 16:02:08 [-0500], Johannes Weiner wrote:
> > how do you define safe? I've been looking for dependencies of
> > __mod_lruvec_state() but found only that the lock is held during the RMW
> > operation with WORKINGSET_NODES idx.
> 
> These stat functions are not allowed to nest, and the executing thread
> cannot migrate to another CPU during the operation, otherwise they
> corrupt the state they're modifying.

If everyone is taking the same lock (like i_pages.xa_lock) then there
will not be two instances updating the same stat. The owner of the
(sleeping)-spinlock will not be migrated to another CPU.

> They are called from interrupt handlers, such as when NR_WRITEBACK is
> decreased. Thus workingset_node_update() must exclude preemption from
> irq handlers on the local CPU.

Do you have an example for a code path to check NR_WRITEBACK?
 
> They rely on IRQ-disabling to also disable CPU migration.
The spinlock disables CPU migration. 

> > >                                            I'm guessing it's because
> > > preemption is disabled and irq handlers are punted to process context.
> > preemption is enabled and IRQ are processed in forced-threaded mode.
> 
> That doesn't sound safe.

Do you have test-case or something I could throw at it and verify that
this still works? So far nothing complains…

Sebastian