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. > 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. Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
