drm/i915/fence: Avoid del_timer_sync() from inside a timer

A fence may be signaled from any context, including from inside a timer.
One example is timer_i915_sw_fence_wake() which is used to provide a
safety-net when waiting on an external fence. If the external fence is
not signaled within a timely fashion, we signal our fence on its behalf,
and so we then may process subsequent fences in the chain from within
that timer context.

Given that dma_i915_sw_fence_wake() may be from inside a timer, we cannot
then use del_timer_sync() as that requires the timer lock for itself. To
circumvent this, while trying to keep the signal propagation as low
latency as possible, move the completion into a worker and use a bit of
atomic switheroo to serialise the timer-callback and the dma-callback.

Testcase: igt/gem_eio/in-flight-external
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170911084135.22903-3-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
index e9e64e8..dfd9588 100644 (file)
--- a/drivers/gpu/drm/i915/Kconfig
+++ b/drivers/gpu/drm/i915/Kconfig
@@ -12,6 +12,7 @@ config DRM_I915
        select DRM_PANEL
        select DRM_MIPI_DSI
        select RELAY
+       select IRQ_WORK
        # i915 depends on ACPI_VIDEO when ACPI is enabled
        # but for select to work, need to select ACPI_VIDEO's dependencies, ick
        select BACKLIGHT_LCD_SUPPORT if ACPI

diff --git a/drivers/gpu/drm/i915/i915_sw_fence.c b/drivers/gpu/drm/i915/i915_sw_fence.c
index f29540f..808ea4d 100644 (file)
--- a/drivers/gpu/drm/i915/i915_sw_fence.c
+++ b/drivers/gpu/drm/i915/i915_sw_fence.c
@@ -9,6 +9,7 @@
 
 #include <linux/slab.h>
 #include <linux/dma-fence.h>
+#include <linux/irq_work.h>
 #include <linux/reservation.h>
 
 #include "i915_sw_fence.h"
@@ -356,31 +357,44 @@ struct i915_sw_dma_fence_cb {
        struct i915_sw_fence *fence;
        struct dma_fence *dma;
        struct timer_list timer;
+       struct irq_work work;
 };
 
 static void timer_i915_sw_fence_wake(unsigned long data)
 {
        struct i915_sw_dma_fence_cb *cb = (struct i915_sw_dma_fence_cb *)data;
+       struct i915_sw_fence *fence;
+
+       fence = xchg(&cb->fence, NULL);
+       if (!fence)
+               return;
 
        pr_warn("asynchronous wait on fence %s:%s:%x timed out\n",
                cb->dma->ops->get_driver_name(cb->dma),
                cb->dma->ops->get_timeline_name(cb->dma),
                cb->dma->seqno);
-       dma_fence_put(cb->dma);
-       cb->dma = NULL;
 
-       i915_sw_fence_complete(cb->fence);
-       cb->timer.function = NULL;
+       i915_sw_fence_complete(fence);
 }
 
 static void dma_i915_sw_fence_wake(struct dma_fence *dma,
                                   struct dma_fence_cb *data)
 {
        struct i915_sw_dma_fence_cb *cb = container_of(data, typeof(*cb), base);
+       struct i915_sw_fence *fence;
+
+       fence = xchg(&cb->fence, NULL);
+       if (fence)
+               i915_sw_fence_complete(fence);
+
+       irq_work_queue(&cb->work);
+}
+
+static void irq_i915_sw_fence_work(struct irq_work *wrk)
+{
+       struct i915_sw_dma_fence_cb *cb = container_of(wrk, typeof(*cb), work);
 
        del_timer_sync(&cb->timer);
-       if (cb->timer.function)
-               i915_sw_fence_complete(cb->fence);
        dma_fence_put(cb->dma);
 
        kfree(cb);
@@ -414,6 +428,7 @@ int i915_sw_fence_await_dma_fence(struct i915_sw_fence *fence,
        __setup_timer(&cb->timer,
                      timer_i915_sw_fence_wake, (unsigned long)cb,
                      TIMER_IRQSAFE);
+       init_irq_work(&cb->work, irq_i915_sw_fence_work);
        if (timeout) {
                cb->dma = dma_fence_get(dma);
                mod_timer(&cb->timer, round_jiffies_up(jiffies + timeout));