[Intel-gfx] [PATCH] drm/i915/gt: Do not add same i915_request to intel_context twice

[Yang, Dong]

I am working on a customized kernel based on 5.4.39,  issue can only reproduced when system facing low memory pressure, and system try to reclaim memory, then wrong double insert i915_reqeust coming  from the i915_gem_shrink() path.

i915_request_enable_breadcrumb+0x136/0x14a
dma_fence_enable_sw_signaling+0x47/0xb0
enable_signaling+0x66/0x80
i915_active_wait+0xc1/0x150
__i915_vma_unbind+0x17/0x1a0
i915_vma_unbind+0x47/0xc0
i915_gem_object_unbind+0x189/0x290
i915_gem_shrink+0x139/0x460
? __pm_runtime_resume+0x53/0x70
i915_gem_shrinker_scan+0x9c/0xb0
do_shrink_slab+0x14f/0x2b0
shrink_slab+0xa7/0x2a0
shrink_node+0xd1/0x410
balance_pgdat+0x2b7/0x500
kswapd+0x1e2/0x3b0

I believe it's not related to the ce->signal_lock,  the lock should works normally.

The i915_request_enable_breadcrumb() can be invoked by several context, like called from ioctl(), from interrupt context, and from memory swap thread, I suggest add a double check before insert i915_request to the list, it's hard to assure valid call from all the paths, but add check&protect can avoid the critical effect,  because add same i915_request twice will trigger a dead loop in signal_irq_work() , and the loop will never break continue the i915_request. hwsp_seqno be changed, and invalid address access error reported followed by system panic.

Thanks,
Dong

-----Original Message-----
From: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> 
Sent: Friday, December 10, 2021 4:51 PM
To: Yang, Dong <dong.yang at intel.com>; intel-gfx at lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915/gt: Do not add same i915_request to intel_context twice


On 10/12/2021 01:31, dong.yang at intel.com wrote:
> From: "Yang, Dong" <dong.yang at intel.com>
> 
> With unknow race condition, the i915_request will be added

What do you mean with unknown here?

> to intel_context list twice, and result in system panic.
> 
> If node alreay exist then do not add it again.

Note the call chains are under ce->signal_lock and protecting from double add AFAICT:

static void insert_breadcrumb(struct i915_request *rq) { ...
	if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
		return;
...
	set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);


bool i915_request_enable_breadcrumb(struct i915_request *rq) { ...
	spin_lock(&ce->signal_lock);
	if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
		insert_breadcrumb(rq);
	spin_unlock(&ce->signal_lock);


void i915_request_cancel_breadcrumb(struct i915_request *rq) { ...
	spin_lock(&ce->signal_lock);
	if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags)) {
		spin_unlock(&ce->signal_lock);
		return;
	}

void intel_context_remove_breadcrumbs(struct intel_context *ce,
				      struct intel_breadcrumbs *b)
{
...
	spin_lock_irqsave(&ce->signal_lock, flags);

	if (list_empty(&ce->signals))
		goto unlock;

	list_for_each_entry_safe(rq, rn, &ce->signals, signal_link) {
		GEM_BUG_ON(!__i915_request_is_complete(rq));
		if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
					&rq->fence.flags))
			continue;

The last one in signal_irq_work is guarded by the __i915_request_is_complete check.

So I think more context is needed on how you found this may be an issue.

Regards,

Tvrtko

> 
> Signed-off-by: Yang, Dong <dong.yang at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c 
> b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index 209cf265bf74..9c7bc060d2ae 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -387,6 +387,9 @@ static void insert_breadcrumb(struct i915_request *rq)
>   		}
>   	}
>   
> +	if (&rq->signal_link == pos)
> +		return;
> +
>   	i915_request_get(rq);
>   	list_add_rcu(&rq->signal_link, pos);
>   	GEM_BUG_ON(!check_signal_order(ce, rq));
>