Re: [Intel-gfx] [PATCH] drm/i915: fix blank screen booting crashes

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Sep 21, 2021 at 04:29:31PM -0700, Lucas De Marchi wrote:
> On Tue, Sep 21, 2021 at 03:55:15PM -0700, Matthew Brost wrote:
> > On Tue, Sep 21, 2021 at 11:46:37AM -0700, Lucas De Marchi wrote:
> > > On Tue, Sep 21, 2021 at 10:43:32AM -0700, Matthew Brost wrote:
> > > > From: Hugh Dickins <hughd@google.com>
> > > >
> > > > 5.15-rc1 crashes with blank screen when booting up on two ThinkPads
> > > > using i915.  Bisections converge convincingly, but arrive at different
> > > > and surprising "culprits", none of them the actual culprit.
> > > >
> > > > netconsole (with init_netconsole() hacked to call i915_init() when
> > > > logging has started, instead of by module_init()) tells the story:
> > > >
> > > > kernel BUG at drivers/gpu/drm/i915/i915_sw_fence.c:245!
> > > > with RSI: ffffffff814d408b pointing to sw_fence_dummy_notify().
> > > > I've been building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y, and that
> > > > function needs to be 4-byte aligned.
> > > >
> > > > v2:
> > > > (Jani Nikula)
> > > >  - Change BUG_ON to WARN_ON
> > > > v3:
> > > > (Jani / Tvrtko)
> > > >  - Short circuit __i915_sw_fence_init on WARN_ON
> > > >
> > > > Fixes: 62eaf0ae217d ("drm/i915/guc: Support request cancellation")
> > > > Signed-off-by: Hugh Dickins <hughd@google.com>
> > > > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > > > Reviewed-by: Matthew Brost <matthew.brost@intel.com>
> > > > ---
> > > > drivers/gpu/drm/i915/gt/intel_context.c |  4 ++--
> > > > drivers/gpu/drm/i915/i915_sw_fence.c    | 17 ++++++++++-------
> > > > 2 files changed, 12 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > > > index ff637147b1a9..e7f78bc7ebfc 100644
> > > > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > > > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > > > @@ -362,8 +362,8 @@ static int __intel_context_active(struct i915_active *active)
> > > > 	return 0;
> > > > }
> > > >
> > > 
> > > > -static int sw_fence_dummy_notify(struct i915_sw_fence *sf,
> > > > -				 enum i915_sw_fence_notify state)
> > > > +static int __i915_sw_fence_call
> > > > +sw_fence_dummy_notify(struct i915_sw_fence *sf, enum i915_sw_fence_notify state)
> > > > {
> > > > 	return NOTIFY_DONE;
> > > > }
> > > > diff --git a/drivers/gpu/drm/i915/i915_sw_fence.c b/drivers/gpu/drm/i915/i915_sw_fence.c
> > > > index c589a681da77..08cea73264e7 100644
> > > > --- a/drivers/gpu/drm/i915/i915_sw_fence.c
> > > > +++ b/drivers/gpu/drm/i915/i915_sw_fence.c
> > > > @@ -13,9 +13,9 @@
> > > > #include "i915_selftest.h"
> > > >
> > > > #if IS_ENABLED(CONFIG_DRM_I915_DEBUG)
> > > > -#define I915_SW_FENCE_BUG_ON(expr) BUG_ON(expr)
> > > > +#define I915_SW_FENCE_WARN_ON(expr) WARN_ON(expr)
> > > > #else
> > > > -#define I915_SW_FENCE_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
> > > > +#define I915_SW_FENCE_WARN_ON(expr) BUILD_BUG_ON_INVALID(expr)
> > > > #endif
> > > >
> > > > static DEFINE_SPINLOCK(i915_sw_fence_lock);
> > > > @@ -129,7 +129,10 @@ static int __i915_sw_fence_notify(struct i915_sw_fence *fence,
> > > > 	i915_sw_fence_notify_t fn;
> > > >
> > > > 	fn = (i915_sw_fence_notify_t)(fence->flags & I915_SW_FENCE_MASK);
> > > > -	return fn(fence, state);
> > > > +	if (likely(fn))
> > > > +		return fn(fence, state);
> > > > +	else
> > > > +		return 0;
> > > 
> > > since the knowledge for these being NULL (or with the wrong alignment)
> > > are in the init/reinit functions,  wouldn't it be better to just add a
> > > fence_nop() and assign it there instead this likely() here?
> > > 
> > 
> > Maybe? I prefer the way it is.
> > 
> > > > }
> > > >
> > > > #ifdef CONFIG_DRM_I915_SW_FENCE_DEBUG_OBJECTS
> > > > @@ -242,9 +245,9 @@ void __i915_sw_fence_init(struct i915_sw_fence *fence,
> > > > 			  const char *name,
> > > > 			  struct lock_class_key *key)
> > > > {
> > > > -	BUG_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK);
> > > > -
> > > > 	__init_waitqueue_head(&fence->wait, name, key);
> > > > +	if (WARN_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK))
> > > > +		return;
> > > 
> > > like:
> > > 	if (WARN_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK))
> > > 		fence->flags = (unsigned long)sw_fence_dummy_notify;
> > > 	else
> > > 		fence->flags = (unsigned long)fn;
> > > 
> > > 
> > > f you return here instead of calling i915_sw_fence_reinit(), aren't you
> > > just going to use uninitialized memory later? At least in the selftests,
> > > which allocate it with kmalloc()... I didn't check others.
> > > 
> > 
> > I don't think so, maybe the fence won't work but it won't blow up
> > either.
> > 
> > > 
> > > For the bug fix we could just add the __aligned(4) and leave the rest to a
> > > separate patch.
> > > 
> > 
> > The bug was sw_fence_dummy_notify in gt/intel_context.c was not 4 byte
> > align which triggered a BUG_ON during boot which blank screened a
> > laptop. Jani / Tvrtko suggested that we make the BUG_ON to WARN_ONs so
> > if someone makes this mistake in the future kernel should boot albiet
> > with a WARNING.
> 
> yes, I understood. But afaics with WARN_ON you are allowing it to
> continue and may be using uninitialized memory later, just causing other
> problems down the line, which may be equally difficult to debug.
> 
> what I suggested is that there is the easy fix to apply to the current
> rcX kernel, adding __aligned(4) to sw_fence_dummy_notify() (patch 1).
> And there is the additional protection being added here (patch 2) which
> is subject to the debate.
> 

Got it. Will post as 2 different patches.

> > 
> > The long term fix is just pull out the I915_SW_FENCE_MASK (stealing bits
> > from a poitner) and we don't have to worry any of this.
> 
> Patch 2 may not even be needed if you're going that route. But we are
> not only protecting against unaligned, but also from code calling
> i915_sw_fence_init() with a NULL fn.
> 

Maybe, I'll just do the proper fix in patch #2 right away.

Matt

> Lucas De Marchi
> 
> > 
> > Matt
> > 
> > > 
> > > Lucas De Marchi
> > > 
> > > > 	fence->flags = (unsigned long)fn;
> > > >
> > > > 	i915_sw_fence_reinit(fence);
> > > > @@ -257,8 +260,8 @@ void i915_sw_fence_reinit(struct i915_sw_fence *fence)
> > > > 	atomic_set(&fence->pending, 1);
> > > > 	fence->error = 0;
> > > >
> > > > -	I915_SW_FENCE_BUG_ON(!fence->flags);
> > > > -	I915_SW_FENCE_BUG_ON(!list_empty(&fence->wait.head));
> > > > +	I915_SW_FENCE_WARN_ON(!fence->flags);
> > > > +	I915_SW_FENCE_WARN_ON(!list_empty(&fence->wait.head));
> > > > }
> > > >
> > > > void i915_sw_fence_commit(struct i915_sw_fence *fence)
> > > > --
> > > > 2.32.0
> > > >