Re: [PATCH 7/9] irq: remove needless lock in takedown_cpu()

[Thomas Gleixner]

On Mon, Apr 25 2022 at 10:57, Pingfan Liu wrote:
> On Thu, Apr 21, 2022 at 06:11:56PM +0200, Thomas Gleixner wrote:
>> > -	irq_lock_sparse();
>> 
>> Not everything is about RCU here. You really need to look at all moving
>> parts:
>> 
>> irq_migrate_all_off_this_cpu() relies on the allocated_irqs bitmap and
>> the sparse tree to be in consistent state, which is only guaranteed when
>> the sparse lock is held.
>> 
>
> For the irq which transfer from active to inactive(disappearing) after
> fetching, desc->lock can serve the sync purpose. In this case,
> irq_lock_sparse() is not needed. For a emergeing irq, I am not sure
> about it.

No, it's required for the free case. The alloc case is
uninteresting. Care to look into the code?

irq_free_descs()
   lock(sparse);
   free_descs();
   bitmap_clear(allocated_irqs, from, cnt);
   unlock_sparse);
 
As free_descs() sets the sparse tree entry to NULL, up to the point
where bitmap_clear() finishes the state is inconsistent.

Now look at irq_migrate_all_off_this_cpu() and figure out what happens
when stop_machine() hits into the inconsistent state.

This can be fixed, but not by making mysterious claims about RCU and
desc->lock.

Thanks,

        tglx