On Sun, Nov 09, 2014 at 01:53:37PM +0400, Konstantin Khlebnikov wrote:
> 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.
>
> Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
> Reported-and-tested-by: Alexander Monakov <amonakov@xxxxxxxxx>
> Cc: Tom Boshoven <tomboshoven@xxxxxxxxx>
> Link: https://devtalk.nvidia.com/default/topic/567297/linux/linux-3-10-driver-crash/
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/acpi/osl.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
> index 9964f70..217713c 100644
> --- a/drivers/acpi/osl.c
> +++ b/drivers/acpi/osl.c
> @@ -436,7 +436,7 @@ static void acpi_os_drop_map_ref(struct acpi_ioremap *map)
> 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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