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 <hu...@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 <hu...@google.com>
> Signed-off-by: Matthew Brost <matthew.br...@intel.com>
> Reviewed-by: Matthew Brost <matthew.br...@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.
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.
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
>
