drm/i915/gt: Acquire ce->active before ce->pin_count/ce->pin_mutex

Similar to commit ac0e331a ("drm/i915: Tighten atomicity of
i915_active_acquire vs i915_active_release") we have the same race of
trying to pin the context underneath a mutex while allowing the
decrement to be atomic outside of that mutex. This leads to the problem
where two threads may simultaneously try to pin the context and the
second not notice that they needed to repin the context.

<2> [198.669621] kernel BUG at drivers/gpu/drm/i915/gt/intel_timeline.c:387!
<4> [198.669703] invalid opcode: 0000 [#1] PREEMPT SMP PTI
<4> [198.669712] CPU: 0 PID: 1246 Comm: gem_exec_create Tainted: G     U  W         5.5.0-rc6-CI-CI_DRM_7755+ #1
<4> [198.669723] Hardware name:  /NUC7i5BNB, BIOS BNKBL357.86A.0054.2017.1025.1822 10/25/2017
<4> [198.669776] RIP: 0010:timeline_advance+0x7b/0xe0 [i915]
<4> [198.669785] Code: 00 48 c7 c2 10 f1 46 a0 48 c7 c7 70 1b 32 a0 e8 bb dd e7 e0 bf 01 00 00 00 e8 d1 af e7 e0 31 f6 bf 09 00 00 00 e8 35 ef d8 e0 <0f> 0b 48 c7 c1 48 fa 49 a0 ba 84 01 00 00 48 c7 c6 10 f1 46 a0 48
<4> [198.669803] RSP: 0018:ffffc900004c3a38 EFLAGS: 00010296
<4> [198.669810] RAX: ffff888270b35140 RBX: ffff88826f32ee00 RCX: 0000000000000006
<4> [198.669818] RDX: 00000000000017c5 RSI: 0000000000000000 RDI: 0000000000000009
<4> [198.669826] RBP: ffffc900004c3a64 R08: 0000000000000000 R09: 0000000000000000
<4> [198.669834] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88826f9b5980
<4> [198.669841] R13: 0000000000000cc0 R14: ffffc900004c3dc0 R15: ffff888253610068
<4> [198.669849] FS:  00007f63e663fe40(0000) GS:ffff888276c00000(0000) knlGS:0000000000000000
<4> [198.669857] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [198.669864] CR2: 00007f171f8e39a8 CR3: 000000026b1f6005 CR4: 00000000003606f0
<4> [198.669872] Call Trace:
<4> [198.669924]  intel_timeline_get_seqno+0x12/0x40 [i915]
<4> [198.669977]  __i915_request_create+0x76/0x5a0 [i915]
<4> [198.670024]  i915_request_create+0x86/0x1c0 [i915]
<4> [198.670068]  i915_gem_do_execbuffer+0xbf2/0x2500 [i915]
<4> [198.670082]  ? __lock_acquire+0x460/0x15d0
<4> [198.670128]  i915_gem_execbuffer2_ioctl+0x11f/0x470 [i915]
<4> [198.670171]  ? i915_gem_execbuffer_ioctl+0x300/0x300 [i915]
<4> [198.670181]  drm_ioctl_kernel+0xa7/0xf0
<4> [198.670188]  drm_ioctl+0x2e1/0x390
<4> [198.670233]  ? i915_gem_execbuffer_ioctl+0x300/0x300 [i915]

Fixes: 84135022 ("drm/i915/gt: Drop mutex serialisation between context pin/unpin")
References: ac0e331a ("drm/i915: Tighten atomicity of i915_active_acquire vs i915_active_release")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200127152829.2842149-1-chris@chris-wilson.co.uk
(cherry picked from commit e5429340)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/gt/intel_context.c

@@ -67,21 +67,18 @@ static int intel_context_active_acquire(struct intel_context *ce)
 {
 	int err;
 
-	err = i915_active_acquire(&ce->active);
-	if (err)
-		return err;
+	__i915_active_acquire(&ce->active);
+
+	if (intel_context_is_barrier(ce))
+		return 0;
 
 	/* Preallocate tracking nodes */
-	if (!intel_context_is_barrier(ce)) {
 	err = i915_active_acquire_preallocate_barrier(&ce->active,
 						      ce->engine);
-		if (err) {
+	if (err)
 		i915_active_release(&ce->active);
-			return err;
-		}
-	}
 
-	return 0;
+	return err;
 }
 
 static void intel_context_active_release(struct intel_context *ce)
@@ -101,13 +98,19 @@ int __intel_context_do_pin(struct intel_context *ce)
 			return err;
 	}
 
-	if (mutex_lock_interruptible(&ce->pin_mutex))
-		return -EINTR;
+	err = i915_active_acquire(&ce->active);
+	if (err)
+		return err;
 
-	if (likely(!atomic_read(&ce->pin_count))) {
+	if (mutex_lock_interruptible(&ce->pin_mutex)) {
+		err = -EINTR;
+		goto out_release;
+	}
+
+	if (likely(!atomic_add_unless(&ce->pin_count, 1, 0))) {
 		err = intel_context_active_acquire(ce);
 		if (unlikely(err))
-			goto err;
+			goto out_unlock;
 
 		err = ce->ops->pin(ce);
 		if (unlikely(err))
@@ -117,18 +120,19 @@ int __intel_context_do_pin(struct intel_context *ce)
 			 ce->ring->head, ce->ring->tail);
 
 		smp_mb__before_atomic(); /* flush pin before it is visible */
+		atomic_inc(&ce->pin_count);
 	}
 
-	atomic_inc(&ce->pin_count);
 	GEM_BUG_ON(!intel_context_is_pinned(ce)); /* no overflow! */
-
-	mutex_unlock(&ce->pin_mutex);
-	return 0;
+	GEM_BUG_ON(i915_active_is_idle(&ce->active));
+	goto out_unlock;
 
 err_active:
 	intel_context_active_release(ce);
-err:
+out_unlock:
 	mutex_unlock(&ce->pin_mutex);
+out_release:
+	i915_active_release(&ce->active);
 	return err;
 }
 

drivers/gpu/drm/i915/i915_active.h

@@ -188,6 +188,12 @@ int i915_active_acquire(struct i915_active *ref);
 bool i915_active_acquire_if_busy(struct i915_active *ref);
 void i915_active_release(struct i915_active *ref);
 
+static inline void __i915_active_acquire(struct i915_active *ref)
+{
+	GEM_BUG_ON(!atomic_read(&ref->count));
+	atomic_inc(&ref->count);
+}
+
 static inline bool
 i915_active_is_idle(const struct i915_active *ref)
 {