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Subject[PATCH v3] drm/i915: Synchronize active and retire callbacks
Date
From: Sultan Alsawaf <sultan@kerneltoast.com>

Active and retire callbacks can run simultaneously, causing panics and
mayhem. The most notable case is with the intel_context_pin/unpin race
that causes ring and page table corruption. In 5.4, this race is more
noticeable because intel_ring_unpin() sets ring->vaddr to NULL and
causes a clean NULL-pointer-dereference panic, but in newer kernels this
race goes unnoticed.

Here is an example of a crash caused by this race on 5.4:
BUG: unable to handle page fault for address: 0000000000003448
RIP: 0010:gen8_emit_flush_render+0x163/0x190
Call Trace:
execlists_request_alloc+0x25/0x40
__i915_request_create+0x1f4/0x2c0
i915_request_create+0x71/0xc0
i915_gem_do_execbuffer+0xb98/0x1a80
? preempt_count_add+0x68/0xa0
? _raw_spin_lock+0x13/0x30
? _raw_spin_unlock+0x16/0x30
i915_gem_execbuffer2_ioctl+0x1de/0x3c0
? i915_gem_busy_ioctl+0x7f/0x1d0
? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
drm_ioctl_kernel+0xb2/0x100
drm_ioctl+0x209/0x360
? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
ksys_ioctl+0x87/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x4e/0x150
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Protect the active and retire callbacks with their own mutex to prevent
them from running at the same time as one another. This change makes
the i915_active_may_sleep() functionality a redundant for its only user,
so clean that out as well since it's no longer needed.

Fixes: 12c255b5dad1 ("drm/i915: Provide an i915_active.acquire callback")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
v3: Protecting only intel_context_pin/unpin is insufficient because all
the callbacks race with each other, not just those ones. Now, all
callbacks are protected from racing with their counterparts with a
new mutex lock for reduced scope. This isn't as pretty, but it is
unavoidable.

.../gpu/drm/i915/display/intel_frontbuffer.c | 4 +--
drivers/gpu/drm/i915/gem/i915_gem_context.c | 1 -
drivers/gpu/drm/i915/gt/intel_context.c | 1 -
drivers/gpu/drm/i915/gt/intel_engine_pool.c | 1 -
drivers/gpu/drm/i915/gt/intel_timeline.c | 1 -
drivers/gpu/drm/i915/i915_active.c | 30 ++++++++++++++-----
drivers/gpu/drm/i915/i915_active_types.h | 6 +---
drivers/gpu/drm/i915/i915_vma.c | 1 -
8 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.c b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
index 6cb02c912acc..766325948b5b 100644
--- a/drivers/gpu/drm/i915/display/intel_frontbuffer.c
+++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
@@ -206,7 +206,6 @@ static int frontbuffer_active(struct i915_active *ref)
return 0;
}

-__i915_active_call
static void frontbuffer_retire(struct i915_active *ref)
{
struct intel_frontbuffer *front =
@@ -254,8 +253,7 @@ intel_frontbuffer_get(struct drm_i915_gem_object *obj)
kref_init(&front->ref);
atomic_set(&front->bits, 0);
i915_active_init(&front->write,
- frontbuffer_active,
- i915_active_may_sleep(frontbuffer_retire));
+ frontbuffer_active, frontbuffer_retire);

spin_lock(&i915->fb_tracking.lock);
if (rcu_access_pointer(obj->frontbuffer)) {
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 68326ad3b2e0..31791b4ae086 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1057,7 +1057,6 @@ struct context_barrier_task {
void *data;
};

-__i915_active_call
static void cb_retire(struct i915_active *base)
{
struct context_barrier_task *cb = container_of(base, typeof(*cb), base);
diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index aea992e46c42..964e32cf5856 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -222,7 +222,6 @@ static void __ring_retire(struct intel_ring *ring)
i915_active_release(&ring->vma->active);
}

-__i915_active_call
static void __intel_context_retire(struct i915_active *active)
{
struct intel_context *ce = container_of(active, typeof(*ce), active);
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pool.c b/drivers/gpu/drm/i915/gt/intel_engine_pool.c
index 397186818305..60dde6978511 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_pool.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_pool.c
@@ -61,7 +61,6 @@ static int pool_active(struct i915_active *ref)
return 0;
}

-__i915_active_call
static void pool_retire(struct i915_active *ref)
{
struct intel_engine_pool_node *node =
diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
index 91debbc97c9a..ecd120e1cf2a 100644
--- a/drivers/gpu/drm/i915/gt/intel_timeline.c
+++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
@@ -131,7 +131,6 @@ static void __idle_cacheline_free(struct intel_timeline_cacheline *cl)
kfree_rcu(cl, rcu);
}

-__i915_active_call
static void __cacheline_retire(struct i915_active *active)
{
struct intel_timeline_cacheline *cl =
diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index c4048628188a..165216aee122 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -148,8 +148,15 @@ __active_retire(struct i915_active *ref)
spin_unlock_irqrestore(&ref->tree_lock, flags);

/* After the final retire, the entire struct may be freed */
- if (ref->retire)
- ref->retire(ref);
+ if (ref->retire) {
+ if (ref->active) {
+ mutex_lock(&ref->callback_lock);
+ ref->retire(ref);
+ mutex_unlock(&ref->callback_lock);
+ } else {
+ ref->retire(ref);
+ }
+ }

/* ... except if you wait on it, you must manage your own references! */
wake_up_var(ref);
@@ -281,15 +288,15 @@ void __i915_active_init(struct i915_active *ref,
struct lock_class_key *mkey,
struct lock_class_key *wkey)
{
- unsigned long bits;
-
debug_active_init(ref);

ref->flags = 0;
ref->active = active;
- ref->retire = ptr_unpack_bits(retire, &bits, 2);
- if (bits & I915_ACTIVE_MAY_SLEEP)
+ ref->retire = retire;
+ if (ref->active && ref->retire) {
+ mutex_init(&ref->callback_lock);
ref->flags |= I915_ACTIVE_RETIRE_SLEEPS;
+ }

spin_lock_init(&ref->tree_lock);
ref->tree = RB_ROOT;
@@ -428,8 +435,15 @@ int i915_active_acquire(struct i915_active *ref)
return err;

if (likely(!i915_active_acquire_if_busy(ref))) {
- if (ref->active)
- err = ref->active(ref);
+ if (ref->active) {
+ if (ref->retire) {
+ mutex_lock(&ref->callback_lock);
+ err = ref->active(ref);
+ mutex_unlock(&ref->callback_lock);
+ } else {
+ err = ref->active(ref);
+ }
+ }
if (!err) {
spin_lock_irq(&ref->tree_lock); /* __active_retire() */
debug_active_activate(ref);
diff --git a/drivers/gpu/drm/i915/i915_active_types.h b/drivers/gpu/drm/i915/i915_active_types.h
index 6360c3e4b765..19b06c64c017 100644
--- a/drivers/gpu/drm/i915/i915_active_types.h
+++ b/drivers/gpu/drm/i915/i915_active_types.h
@@ -24,11 +24,6 @@ struct i915_active_fence {

struct active_node;

-#define I915_ACTIVE_MAY_SLEEP BIT(0)
-
-#define __i915_active_call __aligned(4)
-#define i915_active_may_sleep(fn) ptr_pack_bits(&(fn), I915_ACTIVE_MAY_SLEEP, 2)
-
struct i915_active {
atomic_t count;
struct mutex mutex;
@@ -45,6 +40,7 @@ struct i915_active {

int (*active)(struct i915_active *ref);
void (*retire)(struct i915_active *ref);
+ struct mutex callback_lock;

struct work_struct work;

diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 08699fa069aa..0f73beb586ab 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -93,7 +93,6 @@ static int __i915_vma_active(struct i915_active *ref)
return i915_vma_tryget(active_to_vma(ref)) ? 0 : -ENOENT;
}

-__i915_active_call
static void __i915_vma_retire(struct i915_active *ref)
{
i915_vma_put(active_to_vma(ref));
--
2.26.0
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