Quoting Chris Wilson (2020-07-13 14:17:38)
> Preempt-to-busy introduces various fascinating complications in that the
> requests may complete as we are unsubmitting them from HW. As they may
> then signal after unsubmission, we may find ourselves having to cleanup
> the signaling request from within the signaling callback. This causes us
> to recurse onto the same i915_request.lock.
>
> However, if the request is already signaled (as it will be before we
> enter the signal callbacks), we know we can skip the signaling of that
> request during submission, neatly evading the spinlock recursion.
>
> unsubmit(ve.rq0) # timeslice expiration or other preemption
> -> virtual_submit_request(ve.rq0)
> dma_fence_signal(ve.rq0) # request completed before preemption ack
> -> submit_notify(ve.rq1)
> -> virtual_submit_request(ve.rq1) # sees that we have completed ve.rq0
> -> __i915_request_submit(ve.rq0)
>
> [ 264.210142] BUG: spinlock recursion on CPU#2, sample_multi_tr/2093
> [ 264.210150] lock: 0xffff9efd6ac55080, .magic: dead4ead, .owner: sample_multi_tr/2093, .owner_cpu: 2
> [ 264.210155] CPU: 2 PID: 2093 Comm: sample_multi_tr Tainted: G U
> [ 264.210158] Hardware name: Intel Corporation CoffeeLake Client Platform/CoffeeLake S UDIMM RVP, BIOS CNLSFWR1.R00.X212.B01.1909060036 09/06/2019
> [ 264.210160] Call Trace:
> [ 264.210167] dump_stack+0x98/0xda
> [ 264.210174] spin_dump.cold+0x24/0x3c
> [ 264.210178] do_raw_spin_lock+0x9a/0xd0
> [ 264.210184] _raw_spin_lock_nested+0x6a/0x70
> [ 264.210314] __i915_request_submit+0x10a/0x3c0 [i915]
> [ 264.210415] virtual_submit_request+0x9b/0x380 [i915]
> [ 264.210516] submit_notify+0xaf/0x14c [i915]
> [ 264.210602] __i915_sw_fence_complete+0x8a/0x230 [i915]
> [ 264.210692] i915_sw_fence_complete+0x2d/0x40 [i915]
> [ 264.210762] __dma_i915_sw_fence_wake+0x19/0x30 [i915]
> [ 264.210767] dma_fence_signal_locked+0xb1/0x1c0
> [ 264.210772] dma_fence_signal+0x29/0x50
> [ 264.210871] i915_request_wait+0x5cb/0x830 [i915]
> [ 264.210876] ? dma_resv_get_fences_rcu+0x294/0x5d0
> [ 264.210974] i915_gem_object_wait_fence+0x2f/0x40 [i915]
> [ 264.211084] i915_gem_object_wait+0xce/0x400 [i915]
> [ 264.211178] i915_gem_wait_ioctl+0xff/0x290 [i915]
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Fixes: 22b7a426bbe1 ("drm/i915/execlists: Preempt-to-busy")
> References: 6d06779e8672 ("drm/i915: Load balancing across a virtual engine")
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: "Nayana, Venkata Ramana" <venkata.ramana.nayana@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.4+
> ---
> drivers/gpu/drm/i915/i915_request.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 3bb7320249ae..9b74a1bea5db 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -560,9 +560,7 @@ bool __i915_request_submit(struct i915_request *request)
> engine->serial++;
> result = true;
>
> -xfer: /* We may be recursing from the signal callback of another i915 fence */
> - spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
> -
> +xfer:
> if (!test_and_set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags)) {
> list_move_tail(&request->sched.link, &engine->active.requests);
> clear_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
> @@ -570,12 +568,19 @@ bool __i915_request_submit(struct i915_request *request)
> }
> GEM_BUG_ON(!llist_empty(&request->execute_cb));
>
> - if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
> - !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &request->fence.flags) &&
> - !i915_request_enable_breadcrumb(request))
> - intel_engine_signal_breadcrumbs(engine);
> + /* We may be recursing from the signal callback of another i915 fence */
> + if (!i915_request_signaled(request)) {
> + spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
> +
> + if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> + &request->fence.flags) &&
> + !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> + &request->fence.flags) &&
> + !i915_request_enable_breadcrumb(request))
> + intel_engine_signal_breadcrumbs(engine);
Hmm.
[ 68.742086] kworker/-32 3d.s4 65523842us : i915_request_enable_breadcrumb.cold: i915_request_enable_breadcrumb:345 GEM_BUG_ON(test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
So will take some massaging of i915_request_enable_breadcrumb() as well,
at which point I wonder if we can remove the request->lock from here
entirely.
-Chris
