drm/i915/gt: Mark up racy check of breadcrumb irq enabled

We control b->irq_enabled inside the b->irq_lock, but we check before
entering the spinlock whether or not the interrupt is currently
unmasked.

[ 1511.735208] BUG: KCSAN: data-race in __intel_breadcrumbs_disarm_irq [i915] / intel_engine_disarm_breadcrumbs [i915]
[ 1511.735231]
[ 1511.735242] write to 0xffff8881f75fc214 of 1 bytes by interrupt on cpu 2:
[ 1511.735440]  __intel_breadcrumbs_disarm_irq+0x4b/0x160 [i915]
[ 1511.735635]  signal_irq_work+0x337/0x710 [i915]
[ 1511.735652]  irq_work_run_list+0xd7/0x110
[ 1511.735666]  irq_work_run+0x1d/0x50
[ 1511.735681]  smp_irq_work_interrupt+0x21/0x30
[ 1511.735701]  irq_work_interrupt+0xf/0x20
[ 1511.735722]  __do_softirq+0x6f/0x206
[ 1511.735736]  irq_exit+0xcd/0xe0
[ 1511.735756]  do_IRQ+0x44/0xc0
[ 1511.735773]  ret_from_intr+0x0/0x1c
[ 1511.735787]  schedule+0x0/0xb0
[ 1511.735803]  worker_thread+0x194/0x670
[ 1511.735823]  kthread+0x19a/0x1e0
[ 1511.735837]  ret_from_fork+0x1f/0x30
[ 1511.735848]
[ 1511.735867] read to 0xffff8881f75fc214 of 1 bytes by task 432 on cpu 1:
[ 1511.736068]  intel_engine_disarm_breadcrumbs+0x22/0x80 [i915]
[ 1511.736263]  __engine_park+0x107/0x5d0 [i915]
[ 1511.736453]  ____intel_wakeref_put_last+0x44/0x90 [i915]
[ 1511.736648]  __intel_wakeref_put_last+0x5a/0x70 [i915]
[ 1511.736842]  intel_context_exit_engine+0xf2/0x100 [i915]
[ 1511.737044]  i915_request_retire+0x6b2/0x770 [i915]
[ 1511.737244]  retire_requests+0x7a/0xd0 [i915]
[ 1511.737438]  intel_gt_retire_requests_timeout+0x3a7/0x6f0 [i915]
[ 1511.737633]  i915_drop_caches_set+0x1e7/0x260 [i915]
[ 1511.737650]  simple_attr_write+0xfa/0x110
[ 1511.737665]  full_proxy_write+0x94/0xc0
[ 1511.737679]  __vfs_write+0x4b/0x90
[ 1511.737697]  vfs_write+0xfc/0x280
[ 1511.737718]  ksys_write+0x78/0x100
[ 1511.737732]  __x64_sys_write+0x44/0x60
[ 1511.737751]  do_syscall_64+0x6e/0x2c0
[ 1511.737769]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408092916.5355-1-chris@chris-wilson.co.uk

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

@@ -64,7 +64,7 @@ static void __intel_breadcrumbs_disarm_irq(struct intel_breadcrumbs *b)
 	if (!--b->irq_enabled)
 		irq_disable(engine);
 
-	b->irq_armed = false;
+	WRITE_ONCE(b->irq_armed, false);
 	intel_gt_pm_put_async(engine->gt);
 }
 
@@ -73,7 +73,7 @@ void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
 	unsigned long flags;
 
-	if (!b->irq_armed)
+	if (!READ_ONCE(b->irq_armed))
 		return;
 
 	spin_lock_irqsave(&b->irq_lock, flags);
@@ -233,7 +233,7 @@ static bool __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
 	 * which we can add a new waiter and avoid the cost of re-enabling
 	 * the irq.
 	 */
-	b->irq_armed = true;
+	WRITE_ONCE(b->irq_armed, true);
 
 	/*
 	 * Since we are waiting on a request, the GPU should be busy