drm/i915/gt: Mark up the racy read of execlists->context_tag

Since we are using bitops on context_tag to allow us to reserve and
release inflight tags concurrently, the scan for the next bit is
intentionally racy.

[  516.446854] BUG: KCSAN: data-race in execlists_schedule_in.isra.0 [i915] / execlists_schedule_out [i915]
[  516.446874]
[  516.446886] write (marked) to 0xffff8881f7644048 of 8 bytes by interrupt on cpu 2:
[  516.447076]  execlists_schedule_out+0x538/0x6a0 [i915]
[  516.447263]  process_csb+0x10b/0x3d0 [i915]
[  516.447449]  execlists_submission_tasklet+0x30/0x170 [i915]
[  516.447468]  tasklet_action_common.isra.0+0x42/0x90
[  516.447484]  __do_softirq+0xc8/0x206
[  516.447498]  irq_exit+0xcd/0xe0
[  516.447516]  do_IRQ+0x44/0xc0
[  516.447535]  ret_from_intr+0x0/0x1c
[  516.447550]  cpuidle_enter_state+0x199/0x400
[  516.447572]  cpuidle_enter+0x50/0x90
[  516.447587]  do_idle+0x197/0x1e0
[  516.447600]  cpu_startup_entry+0x14/0x20
[  516.447619]  start_secondary+0xf9/0x130
[  516.447643]  secondary_startup_64+0xa4/0xb0
[  516.447655]
[  516.447671] read to 0xffff8881f7644048 of 8 bytes by task 460 on cpu 1:
[  516.447863]  execlists_schedule_in.isra.0+0x3cf/0x5a0 [i915]
[  516.448064]  execlists_dequeue+0xf8f/0x1690 [i915]
[  516.448252]  __execlists_submission_tasklet+0x48/0x60 [i915]
[  516.448440]  execlists_submit_request+0x2e2/0x310 [i915]
[  516.448634]  submit_notify+0x8f/0xc8 [i915]
[  516.448820]  __i915_sw_fence_complete+0x61/0x420 [i915]
[  516.449005]  i915_sw_fence_complete+0x58/0x80 [i915]
[  516.449208]  i915_sw_fence_commit+0x16/0x20 [i915]
[  516.449399]  __i915_request_queue+0x60/0x70 [i915]
[  516.449590]  i915_gem_do_execbuffer+0x33f1/0x4a00 [i915]
[  516.449782]  i915_gem_execbuffer2_ioctl+0x2a2/0x550 [i915]
[  516.449800]  drm_ioctl_kernel+0xe9/0x130
[  516.449814]  drm_ioctl+0x27d/0x45e
[  516.449827]  ksys_ioctl+0x89/0xb0
[  516.449842]  __x64_sys_ioctl+0x42/0x60
[  516.449864]  do_syscall_64+0x6e/0x2c0
[  516.449878]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511075722.13483-1-chris@chris-wilson.co.uk

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

@@ -1367,7 +1367,7 @@ __execlists_schedule_in(struct i915_request *rq)
 		ce->lrc.ccid = ce->tag;
 	} else {
 		/* We don't need a strict matching tag, just different values */
-		unsigned int tag = ffs(engine->context_tag);
+		unsigned int tag = ffs(READ_ONCE(engine->context_tag));
 
 		GEM_BUG_ON(tag == 0 || tag >= BITS_PER_LONG);
 		clear_bit(tag - 1, &engine->context_tag);