Re: [igt-dev] [Intel-gfx] [PATCH i-g-t] i915/gem_exec_schedule: Trick semaphores into a GPU hang

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 09/04/2019 14:56, Chris Wilson wrote:
> If we have two tasks running on xcs0 and xcs1 independently, but who
> queue subsequent work onto rcs, we may insert semaphores before the rcs
> work and pick unwisely which task to run first. To maximise throughput,
> we want to run on rcs whichever task is ready first. Conversely, if we
> pick wrongly that can be used to trigger a GPU hang with unaware
> userspace.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   tests/i915/gem_exec_schedule.c | 61 ++++++++++++++++++++++++++++++++++
>   1 file changed, 61 insertions(+)
> 
> diff --git a/tests/i915/gem_exec_schedule.c b/tests/i915/gem_exec_schedule.c
> index 3df319bcc..d6f109540 100644
> --- a/tests/i915/gem_exec_schedule.c
> +++ b/tests/i915/gem_exec_schedule.c
> @@ -404,6 +404,65 @@ static void semaphore_userlock(int i915)
>   	igt_spin_batch_free(i915, spin);
>   }
>   
> +static void semaphore_codependency(int i915)
> +{
> +	struct {
> +		igt_spin_t *xcs, *rcs;
> +	} task[2];
> +	unsigned int engine;
> +	int i;
> +
> +	/*
> +	 * Consider two tasks, task A runs on (xcs0, rcs0) and task B
> +	 * on (xcs1, rcs0). That is they must both run a dependent
> +	 * batch on rcs0, after first running in parallel on separate
> +	 * engines. To maximise throughput, we want the shorter xcs task
> +	 * to start on rcs first. However, if we insert semaphores we may
> +	 * pick wrongly and end up running the requests in the least
> +	 * optimal order.
> +	 */
> +
> +	i = 0;
> +	for_each_physical_engine(i915, engine) {
> +		uint32_t ctx;
> +
> +		if (engine == I915_EXEC_RENDER)
> +			continue;
> +
> +		ctx = gem_context_create(i915);
> +
> +		task[i].xcs =
> +			__igt_spin_batch_new(i915,
> +					     .ctx = ctx,
> +					     .engine = engine,
> +					     .flags = IGT_SPIN_POLL_RUN);
> +		igt_spin_busywait_until_running(task[i].xcs);
> +
> +		/* Common rcs tasks will be queued in FIFO */
> +		task[i].rcs =
> +			__igt_spin_batch_new(i915,
> +					     .ctx = ctx,
> +					     .engine = I915_EXEC_RENDER,
> +					     .dependency = task[i].xcs->handle);
> +
> +		gem_context_destroy(i915, ctx);
> +
> +		if (++i == ARRAY_SIZE(task))
> +			break;
> +	}
> +	igt_require(i == ARRAY_SIZE(task));
> +
> +	/* Since task[0] was queued first, it will be first in queue for rcs */
> +	igt_spin_batch_end(task[1].xcs);
> +	igt_spin_batch_end(task[1].rcs);
> +	gem_sync(i915, task[1].rcs->handle); /* to hang if task[0] hogs rcs */
> +
> +	for (i = 0; i < ARRAY_SIZE(task); i++) {
> +		igt_spin_batch_free(i915, task[i].xcs);
> +		igt_spin_batch_free(i915, task[i].rcs);
> +	}
> +}
> +
>   static void reorder(int fd, unsigned ring, unsigned flags)
>   #define EQUAL 1
>   {
> @@ -1393,6 +1452,8 @@ igt_main
>   
>   		igt_subtest("semaphore-user")
>   			semaphore_userlock(fd);
> +		igt_subtest("semaphore-codependency")
> +			semaphore_codependency(fd);
>   
>   		igt_subtest("smoketest-all")
>   			smoketest(fd, ALL_ENGINES, 30);
> 

Just need can_store_dword for code correctness before IGT_SPIN_POLL_RUN 
and with that:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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