Re: [PATCH] nouveau: offload fence uevents work to workqueue

[Danilo Krummrich]

On 1/29/24 02:50, Dave Airlie wrote:

From: Dave Airlie <airl...@redhat.com>

This should break the deadlock between the fctx lock and the irq lock.

This offloads the processing off the work from the irq into a workqueue.

Signed-off-by: Dave Airlie <airl...@redhat.com>

Nouveau's scheduler uses a dedicated wq, hence from this perspective it's
safe deferring fence signalling to the kernel global wq. However, I wonder
if we could create deadlocks by building dependency chains into other
drivers / kernel code that, by chance, makes use of the kernel global wq as
well.

Admittedly, even if, it's gonna be extremely unlikely given that
WQ_MAX_ACTIVE == 512. But maybe it'd be safer to use a dedicated wq.

Also, do we need to CC stable?

---
  drivers/gpu/drm/nouveau/nouveau_fence.c | 24 ++++++++++++++++++------
  drivers/gpu/drm/nouveau/nouveau_fence.h |  1 +
  2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c 
b/drivers/gpu/drm/nouveau/nouveau_fence.c
index ca762ea55413..93f08f9479d8 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -103,6 +103,7 @@ nouveau_fence_context_kill(struct nouveau_fence_chan *fctx, 
int error)
  void
  nouveau_fence_context_del(struct nouveau_fence_chan *fctx)
  {
+       cancel_work_sync(&fctx->uevent_work);
        nouveau_fence_context_kill(fctx, 0);
        nvif_event_dtor(&fctx->event);
        fctx->dead = 1;
@@ -145,12 +146,13 @@ nouveau_fence_update(struct nouveau_channel *chan, struct 
nouveau_fence_chan *fc
        return drop;
  }

-static int

-nouveau_fence_wait_uevent_handler(struct nvif_event *event, void *repv, u32 
repc)
+static void
+nouveau_fence_uevent_work(struct work_struct *work)
  {
-       struct nouveau_fence_chan *fctx = container_of(event, typeof(*fctx), 
event);
+       struct nouveau_fence_chan *fctx = container_of(work, struct 
nouveau_fence_chan,
+                                                      uevent_work);
        unsigned long flags;
-       int ret = NVIF_EVENT_KEEP;
+       int drop = 0;

spin_lock_irqsave(&fctx->lock, flags);

        if (!list_empty(&fctx->pending)) {
@@ -160,11 +162,20 @@ nouveau_fence_wait_uevent_handler(struct nvif_event 
*event, void *repv, u32 repc
                fence = list_entry(fctx->pending.next, typeof(*fence), head);
                chan = rcu_dereference_protected(fence->channel, 
lockdep_is_held(&fctx->lock));
                if (nouveau_fence_update(chan, fctx))
-                       ret = NVIF_EVENT_DROP;
+                       drop = 1;
        }
+       if (drop)
+               nvif_event_block(&fctx->event);
+
        spin_unlock_irqrestore(&fctx->lock, flags);
+}

- return ret;

+static int
+nouveau_fence_wait_uevent_handler(struct nvif_event *event, void *repv, u32 
repc)
+{
+       struct nouveau_fence_chan *fctx = container_of(event, typeof(*fctx), 
event);
+       schedule_work(&fctx->uevent_work);
+       return NVIF_EVENT_KEEP;
  }

void

@@ -178,6 +189,7 @@ nouveau_fence_context_new(struct nouveau_channel *chan, 
struct nouveau_fence_cha
        } args;
        int ret;

+ INIT_WORK(&fctx->uevent_work, nouveau_fence_uevent_work);

        INIT_LIST_HEAD(&fctx->flip);
        INIT_LIST_HEAD(&fctx->pending);
        spin_lock_init(&fctx->lock);
diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h 
b/drivers/gpu/drm/nouveau/nouveau_fence.h
index 64d33ae7f356..8bc065acfe35 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.h
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
@@ -44,6 +44,7 @@ struct nouveau_fence_chan {
        u32 context;
        char name[32];

+ struct work_struct uevent_work;

        struct nvif_event event;
        int notify_ref, dead, killed;
  };