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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > Reviewed-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > ---
> > > 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.
