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Subject[PATCH 3.16 127/148] ACPI / osl: speedup grace period in acpi_os_map_cleanup
3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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-crash/
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);
}
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