Hi, Rafael > From: rjwyso...@gmail.com [mailto:rjwyso...@gmail.com] 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.zh...@intel.com> 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
