Re: [RFC] drm/i915: GEM_WARN_ON considered harmful

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 20 Sep 2018, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
> Ping!
>
> Any comments here?
>
> Main goal was to allow GEM_WARN_ON as a statement, plus also protect 
> uses in if statements, which there are some who I think don't expect the 
> branch to completely disappear.

I've said before I don't like the conditional early returns vanishing
depending on config options, but I've been shot down. I think this patch
is an improvement.


BR,
Jani.


>
> Regards,
>
> Tvrtko
>
> On 07/09/2018 12:53, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> 
>> GEM_WARN_ON currently has dangerous semantics where it is completely
>> compiled out on !GEM_DEBUG builds. This can leave users who expect it to
>> be more like a WARN_ON, just without a warning in non-debug builds, in
>> complete ignorance.
>> 
>> Another gotcha with it is that it cannot be used as a statement. Which is
>> again different from a standard kernel WARN_ON.
>> 
>> This patch fixes both problems by making it behave as one would expect.
>> 
>> It can now be used both as an expression and as statement, and also the
>> condition evaluates properly in all builds - code under the conditional
>> will therefore not unexpectedly disappear.
>> 
>> To satisfy call sites which really want the code under the conditional to
>> completely disappear, we add GEM_DEBUG_WARN_ON and convert some of the
>> callers to it. This one can also be used as both expression and statement.
>> 
>>  From the above it follows GEM_DEBUG_WARN_ON should be used in situations
>> where we are certain the condition will be hit during development, but at
>> a place in code where error can be handled to the benefit of not crashing
>> the machine.
>> 
>> GEM_WARN_ON on the other hand should be used where condition may happen in
>> production and we just want to distinguish the level of debugging output
>> emitted between the production and debug build.
>> 
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>> Cc: Matthew Auld <matthew.auld@intel.com>
>> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
>> ---
>> Quickly put together and compile tested only!
>> ---
>>   drivers/gpu/drm/i915/i915_gem.h          | 4 +++-
>>   drivers/gpu/drm/i915/i915_vma.c          | 8 ++++----
>>   drivers/gpu/drm/i915/intel_engine_cs.c   | 8 ++++----
>>   drivers/gpu/drm/i915/intel_lrc.c         | 8 ++++----
>>   drivers/gpu/drm/i915/intel_workarounds.c | 2 +-
>>   5 files changed, 16 insertions(+), 14 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/i915_gem.h b/drivers/gpu/drm/i915/i915_gem.h
>> index 599c4f6eb1ea..b0e4b976880c 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.h
>> +++ b/drivers/gpu/drm/i915/i915_gem.h
>> @@ -47,17 +47,19 @@ struct drm_i915_private;
>>   #define GEM_DEBUG_DECL(var) var
>>   #define GEM_DEBUG_EXEC(expr) expr
>>   #define GEM_DEBUG_BUG_ON(expr) GEM_BUG_ON(expr)
>> +#define GEM_DEBUG_WARN_ON(expr) GEM_WARN_ON(expr)
>>   
>>   #else
>>   
>>   #define GEM_SHOW_DEBUG() (0)
>>   
>>   #define GEM_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
>> -#define GEM_WARN_ON(expr) (BUILD_BUG_ON_INVALID(expr), 0)
>> +#define GEM_WARN_ON(expr) ({ unlikely(!!(expr)); })
>>   
>>   #define GEM_DEBUG_DECL(var)
>>   #define GEM_DEBUG_EXEC(expr) do { } while (0)
>>   #define GEM_DEBUG_BUG_ON(expr)
>> +#define GEM_DEBUG_WARN_ON(expr) ({ BUILD_BUG_ON_INVALID(expr); 0; })
>>   #endif
>>   
>>   #if IS_ENABLED(CONFIG_DRM_I915_TRACE_GEM)
>> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
>> index 31efc971a3a8..82652c3d1bed 100644
>> --- a/drivers/gpu/drm/i915/i915_vma.c
>> +++ b/drivers/gpu/drm/i915/i915_vma.c
>> @@ -305,12 +305,12 @@ int i915_vma_bind(struct i915_vma *vma, enum i915_cache_level cache_level,
>>   	GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
>>   	GEM_BUG_ON(vma->size > vma->node.size);
>>   
>> -	if (GEM_WARN_ON(range_overflows(vma->node.start,
>> -					vma->node.size,
>> -					vma->vm->total)))
>> +	if (GEM_DEBUG_WARN_ON(range_overflows(vma->node.start,
>> +					      vma->node.size,
>> +					      vma->vm->total)))
>>   		return -ENODEV;
>>   
>> -	if (GEM_WARN_ON(!flags))
>> +	if (GEM_DEBUG_WARN_ON(!flags))
>>   		return -EINVAL;
>>   
>>   	bind_flags = 0;
>> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
>> index 10cd051ba29e..8dbdb18b2668 100644
>> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
>> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
>> @@ -273,13 +273,13 @@ intel_engine_setup(struct drm_i915_private *dev_priv,
>>   	BUILD_BUG_ON(MAX_ENGINE_CLASS >= BIT(GEN11_ENGINE_CLASS_WIDTH));
>>   	BUILD_BUG_ON(MAX_ENGINE_INSTANCE >= BIT(GEN11_ENGINE_INSTANCE_WIDTH));
>>   
>> -	if (GEM_WARN_ON(info->class > MAX_ENGINE_CLASS))
>> +	if (GEM_DEBUG_WARN_ON(info->class > MAX_ENGINE_CLASS))
>>   		return -EINVAL;
>>   
>> -	if (GEM_WARN_ON(info->instance > MAX_ENGINE_INSTANCE))
>> +	if (GEM_DEBUG_WARN_ON(info->instance > MAX_ENGINE_INSTANCE))
>>   		return -EINVAL;
>>   
>> -	if (GEM_WARN_ON(dev_priv->engine_class[info->class][info->instance]))
>> +	if (GEM_DEBUG_WARN_ON(dev_priv->engine_class[info->class][info->instance]))
>>   		return -EINVAL;
>>   
>>   	GEM_BUG_ON(dev_priv->engine[id]);
>> @@ -399,7 +399,7 @@ int intel_engines_init(struct drm_i915_private *dev_priv)
>>   		err = -EINVAL;
>>   		err_id = id;
>>   
>> -		if (GEM_WARN_ON(!init))
>> +		if (GEM_DEBUG_WARN_ON(!init))
>>   			goto cleanup;
>>   
>>   		err = init(engine);
>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>> index 19c9c46308e5..a274cc695fa2 100644
>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>> @@ -1681,7 +1681,7 @@ static int intel_init_workaround_bb(struct intel_engine_cs *engine)
>>   	unsigned int i;
>>   	int ret;
>>   
>> -	if (GEM_WARN_ON(engine->id != RCS))
>> +	if (GEM_DEBUG_WARN_ON(engine->id != RCS))
>>   		return -EINVAL;
>>   
>>   	switch (INTEL_GEN(engine->i915)) {
>> @@ -1720,8 +1720,8 @@ static int intel_init_workaround_bb(struct intel_engine_cs *engine)
>>   	 */
>>   	for (i = 0; i < ARRAY_SIZE(wa_bb_fn); i++) {
>>   		wa_bb[i]->offset = batch_ptr - batch;
>> -		if (GEM_WARN_ON(!IS_ALIGNED(wa_bb[i]->offset,
>> -					    CACHELINE_BYTES))) {
>> +		if (GEM_DEBUG_WARN_ON(!IS_ALIGNED(wa_bb[i]->offset,
>> +						  CACHELINE_BYTES))) {
>>   			ret = -EINVAL;
>>   			break;
>>   		}
>> @@ -1730,7 +1730,7 @@ static int intel_init_workaround_bb(struct intel_engine_cs *engine)
>>   		wa_bb[i]->size = batch_ptr - (batch + wa_bb[i]->offset);
>>   	}
>>   
>> -	BUG_ON(batch_ptr - batch > CTX_WA_BB_OBJ_SIZE);
>> +	GEM_BUG_ON(batch_ptr - batch > CTX_WA_BB_OBJ_SIZE);
>>   
>>   	kunmap_atomic(batch);
>>   	if (ret)
>> diff --git a/drivers/gpu/drm/i915/intel_workarounds.c b/drivers/gpu/drm/i915/intel_workarounds.c
>> index 4bcdeaf8d98f..fb746c4b7c38 100644
>> --- a/drivers/gpu/drm/i915/intel_workarounds.c
>> +++ b/drivers/gpu/drm/i915/intel_workarounds.c
>> @@ -941,7 +941,7 @@ struct whitelist {
>>   
>>   static void whitelist_reg(struct whitelist *w, i915_reg_t reg)
>>   {
>> -	if (GEM_WARN_ON(w->count >= RING_MAX_NONPRIV_SLOTS))
>> +	if (GEM_DEBUG_WARN_ON(w->count >= RING_MAX_NONPRIV_SLOTS))
>>   		return;
>>   
>>   	w->reg[w->count++] = reg;
>> 
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-- 
Jani Nikula, Intel Open Source Graphics Center
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