On 05/10/2017 17:24, Daniel Vetter wrote:
On Thu, Oct 05, 2017 at 03:55:19PM +0100, Tvrtko Ursulin wrote:
On 05/10/2017 15:09, Daniel Vetter wrote:
stop_machine is not really a locking primitive we should use, except
when the hw folks tell us the hw is broken and that's the only way to
work around it.
This patch here is just a suggestion for how to fix it up, possible
changes needed to make it actually work:
- Set the nop_submit_request first for _all_ engines, before
proceeding.
- Make sure engine->cancel_requests copes with the possibility that
not all tests have consistently used the new or old version. I dont
think this is a problem, since the same can happen really with the
stop_machine() locking - stop_machine also doesn't give you any kind
of global ordering against other cpu threads, it just makes them
stop.
This patch tries to address the locking snafu from
commit 20e4933c478a1ca694b38fa4ac44d99e659941f5
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Tue Nov 22 14:41:21 2016 +0000
drm/i915: Stop the machine as we install the wedged submit_request handler
Chris said parts of the reasons for going with stop_machine() was that
it's no overhead for the fast-path. But these callbacks use irqsave
spinlocks and do a bunch of MMIO, and rcu_read_lock is _real_ fast.
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 18 +++++-------------
drivers/gpu/drm/i915/i915_gem_request.c | 2 ++
drivers/gpu/drm/i915/selftests/i915_gem_request.c | 2 ++
3 files changed, 9 insertions(+), 13 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ab8c6946fea4..0b260e576b4b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3022,13 +3022,13 @@ static void nop_submit_request(struct drm_i915_gem_request *request)
static void engine_set_wedged(struct intel_engine_cs *engine)
{
+ engine->submit_request = nop_submit_request;
Should this be rcu_assign_pointer?
Those provide additional barriers, needed when you change/allocate the
stuff you're pointing to. We point to immutable functions, so shouldn't be
necessary (and would be confusing imo).
Ah ok. Any barriers then? Or synchronize_rcu implies them all?
+
/* We need to be sure that no thread is running the old callback as
* we install the nop handler (otherwise we would submit a request
- * to hardware that will never complete). In order to prevent this
- * race, we wait until the machine is idle before making the swap
- * (using stop_machine()).
+ * to hardware that will never complete).
*/
- engine->submit_request = nop_submit_request;
+ synchronize_rcu();
Consumers of this are running in irq disabled or softirq. Does this mean we
would need synchronize_rcu_bh? Would either guarantee all tasklets and irq
handlers have exited?
Oh ... tbh I didn't even digg that deep (much less ran this stuff). This
really is an RFC so people with real clue could say whether it has a
chance of working or not.
Looking at rcu docs we don't want _bh variants, since rcu_read_lock should
be safe in even hardirq context. _bh and _sched otoh require that all
critical sections are either in bottom halfs or hardirq context, since
they treat scheduling of those as a grace period.
rcu_read_unlock might schedule (via preempt_enable) so I don't think we
can use them from the fence callbacks.
And _bh is indeed only for softirq while we need hard and soft. So I am
not sure which one we could use.
It sounds to me any would be wrong and if we wanted to drop stop_machine
we would simply have to use nothing. But then we couldn't be certain
there are no more new requests queued after wedged has been set.
Maybe I am missing something, not sure.
Regards,
Tvrtko
Cheers, Daniel
/* Mark all executing requests as skipped */
engine->cancel_requests(engine);
@@ -3041,9 +3041,8 @@ static void engine_set_wedged(struct intel_engine_cs *engine)
intel_engine_last_submit(engine));
}
-static int __i915_gem_set_wedged_BKL(void *data)
+void i915_gem_set_wedged(struct drm_i915_private *i915)
{
- struct drm_i915_private *i915 = data;
struct intel_engine_cs *engine;
enum intel_engine_id id;
@@ -3052,13 +3051,6 @@ static int __i915_gem_set_wedged_BKL(void *data)
set_bit(I915_WEDGED, &i915->gpu_error.flags);
wake_up_all(&i915->gpu_error.reset_queue);
-
- return 0;
-}
-
-void i915_gem_set_wedged(struct drm_i915_private *dev_priv)
-{
- stop_machine(__i915_gem_set_wedged_BKL, dev_priv, NULL);
}
bool i915_gem_unset_wedged(struct drm_i915_private *i915)
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index b100b38f1dd2..ef78a85cb845 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -556,7 +556,9 @@ submit_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
switch (state) {
case FENCE_COMPLETE:
trace_i915_gem_request_submit(request);
+ rcu_read_lock();
request->engine->submit_request(request);
+ rcu_read_unlock();
And _bh for these? Although this already runs with preemption off, but I
guess it is good for documentation.
Regards,
Tvrtko
break;
case FENCE_FREE:
diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_request.c b/drivers/gpu/drm/i915/selftests/i915_gem_request.c
index 78b9f811707f..a999161e8db1 100644
--- a/drivers/gpu/drm/i915/selftests/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/selftests/i915_gem_request.c
@@ -215,7 +215,9 @@ static int igt_request_rewind(void *arg)
}
i915_gem_request_get(vip);
i915_add_request(vip);
+ rcu_read_lock();
request->engine->submit_request(request);
+ rcu_read_unlock();
mutex_unlock(&i915->drm.struct_mutex);
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