RE: [PATCH] ACPI / OSL: Fix rcu synchronization logic

[Zheng, Lv]

Hi, Rafael

> From: rjwysocki@xxxxxxxxx [mailto:rjwysocki@xxxxxxxxx] On Behalf Of Rafael J. Wysocki
> Sent: Tuesday, January 10, 2017 7:35 AM
> Subject: Re: [PATCH] ACPI / OSL: Fix rcu synchronization logic
> 
> On Mon, Jan 9, 2017 at 10:56 AM, Lv Zheng <lv.zheng@xxxxxxxxx> wrote:
> > The rcu synchronization logic is originally provided to protect
> > apei_read()/apei_write() as in the APEI drivers, there is NMI event source
> > requiring non spinlock based synchronization mechanism.
> >
> > After that, ACPI developers think FADT registers may also require same
> > facility, so they moved the RCU stuffs to generic ACPI layer.
> >
> > So now non-task-context ACPI map lookup is only protected by RCU.
> >
> > This triggers problem as acpi_os_map_memory()/acpi_os_unmap_memory() can be
> > used to map/unmap tables as long as to map/unmap ACPI registers. When it is
> > used for the ACPI tables, the caller could invoke this very early. When it
> > is invoked earlier than workqueue_init() and later than
> > check_early_ioremp_leak(), invoking synchronize_rcu_expedited() can cause a
> > kernel hang.
> >
> > Actually this facility is only used to protect non-task-context ACPI map
> > lookup,
> 
> That doesn't sound quite right.
> 
> acpi_os_read/write_memory() use RCU-protected list lookups, so it's
> not just non-task-context AFAICS.

Yes, you are right.

> 
> > and such mappings are only introduced by
> > acpi_os_map_generic_address(). So before it is invoked, there is no need to
> > invoke synchronize_rcu_expedited().
> 
> That said it may be fine to start actually synchronize RCU after
> acpi_os_map_generic_address() has been called for the first time.  I
> need a better (or more detailed) explanation why it is fine, though.
> 

The reason is wrong.
As list lookups are only protected by RCU.

Thanks
Lv