3.16.82-rc1 review patch. If anyone has any objections, please let me know.
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From: Konstantin Khlebnikov koct9i@gmail.com
commit 74b51ee152b6d99e61ba329799a039453fb9438f upstream.
ACPI maintains cache of ioremap regions to speed up operations and access to them from irq context where ioremap() calls aren't allowed. This code abuses synchronize_rcu() on unmap path for synchronization with fast-path in acpi_os_read/write_memory which uses this cache.
Since v3.10 CPUs are allowed to enter idle state even if they have RCU callbacks queued, see commit c0f4dfd4f90f1667d234d21f15153ea09a2eaa66 ("rcu: Make RCU_FAST_NO_HZ take advantage of numbered callbacks"). That change caused problems with nvidia proprietary driver which calls acpi_os_map/unmap_generic_address several times during initialization. Each unmap calls synchronize_rcu and adds significant delay. Totally initialization is slowed for a couple of seconds and that is enough to trigger timeout in hardware, gpu decides to "fell off the bus". Widely spread workaround is reducing "rcu_idle_gp_delay" from 4 to 1 jiffy.
This patch replaces synchronize_rcu() with synchronize_rcu_expedited() which is much faster.
Link: https://devtalk.nvidia.com/default/topic/567297/linux/linux-3-10-driver-cras... Signed-off-by: Konstantin Khlebnikov koct9i@gmail.com Reported-and-tested-by: Alexander Monakov amonakov@gmail.com Reviewed-by: Paul E. McKenney paulmck@linux.vnet.ibm.com Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/acpi/osl.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/acpi/osl.c +++ b/drivers/acpi/osl.c @@ -425,7 +425,7 @@ static void acpi_os_drop_map_ref(struct static void acpi_os_map_cleanup(struct acpi_ioremap *map) { if (!map->refcount) { - synchronize_rcu(); + synchronize_rcu_expedited(); acpi_unmap(map->phys, map->virt); kfree(map); }