drm/i915: Fix nesting of filelist_mutex vs struct_mutex in i915_ppgtt_info

An unlikely ABBA deadlock in debugfs that no one has reported.

[  284.922349] ======================================================
[  284.922355] [ INFO: possible circular locking dependency detected ]
[  284.922361] 4.8.0-rc2+ #430 Tainted: G        W
[  284.922366] -------------------------------------------------------
[  284.922371] cat/1197 is trying to acquire lock:
[  284.922376]  (&dev->filelist_mutex){+.+...}, at: [<ffffffffa0055ba2>] i915_ppgtt_info+0x82/0x390 [i915]
[  284.922423]
[  284.922423] but task is already holding lock:
[  284.922429]  (&dev->struct_mutex){+.+.+.}, at: [<ffffffffa0055b55>] i915_ppgtt_info+0x35/0x390 [i915]
[  284.922465]
[  284.922465] which lock already depends on the new lock.
[  284.922465]
[  284.922471]
[  284.922471] the existing dependency chain (in reverse order) is:
[  284.922477]
-> #1 (&dev->struct_mutex){+.+.+.}:
[  284.922493]        [<ffffffff81087710>] lock_acquire+0x60/0x80
[  284.922505]        [<ffffffff8143e96f>] mutex_lock_nested+0x5f/0x360
[  284.922520]        [<ffffffffa004f877>] print_context_stats+0x37/0xf0 [i915]
[  284.922549]        [<ffffffffa00535f5>] i915_gem_object_info+0x265/0x490 [i915]
[  284.922581]        [<ffffffff81144491>] seq_read+0xe1/0x3b0
[  284.922592]        [<ffffffff811f77b3>] full_proxy_read+0x83/0xb0
[  284.922604]        [<ffffffff8111ba03>] __vfs_read+0x23/0x110
[  284.922616]        [<ffffffff8111c9b9>] vfs_read+0x89/0x110
[  284.922626]        [<ffffffff8111dbf4>] SyS_read+0x44/0xa0
[  284.922636]        [<ffffffff81442be9>] entry_SYSCALL_64_fastpath+0x1c/0xac
[  284.922648]
-> #0 (&dev->filelist_mutex){+.+...}:
[  284.922667]        [<ffffffff810871fc>] __lock_acquire+0x10fc/0x1270
[  284.922678]        [<ffffffff81087710>] lock_acquire+0x60/0x80
[  284.922689]        [<ffffffff8143e96f>] mutex_lock_nested+0x5f/0x360
[  284.922701]        [<ffffffffa0055ba2>] i915_ppgtt_info+0x82/0x390 [i915]
[  284.922729]        [<ffffffff81144491>] seq_read+0xe1/0x3b0
[  284.922739]        [<ffffffff811f77b3>] full_proxy_read+0x83/0xb0
[  284.922750]        [<ffffffff8111ba03>] __vfs_read+0x23/0x110
[  284.922761]        [<ffffffff8111c9b9>] vfs_read+0x89/0x110
[  284.922771]        [<ffffffff8111dbf4>] SyS_read+0x44/0xa0
[  284.922781]        [<ffffffff81442be9>] entry_SYSCALL_64_fastpath+0x1c/0xac
[  284.922793]
[  284.922793] other info that might help us debug this:
[  284.922793]
[  284.922809]  Possible unsafe locking scenario:
[  284.922809]
[  284.922818]        CPU0                    CPU1
[  284.922825]        ----                    ----
[  284.922831]   lock(&dev->struct_mutex);
[  284.922842]                                lock(&dev->filelist_mutex);
[  284.922854]                                lock(&dev->struct_mutex);
[  284.922865]   lock(&dev->filelist_mutex);
[  284.922875]
[  284.922875]  *** DEADLOCK ***
[  284.922875]
[  284.922888] 3 locks held by cat/1197:
[  284.922895]  #0:  (debugfs_srcu){......}, at: [<ffffffff811f7730>] full_proxy_read+0x0/0xb0
[  284.922919]  #1:  (&p->lock){+.+.+.}, at: [<ffffffff811443e8>] seq_read+0x38/0x3b0
[  284.922942]  #2:  (&dev->struct_mutex){+.+.+.}, at: [<ffffffffa0055b55>] i915_ppgtt_info+0x35/0x390 [i915]
[  284.922983]

Fixes: 1d2ac403 ("drm: Protect dev->filelist with its own mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822132820.21725-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2270,10 +2270,13 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
 	struct drm_i915_private *dev_priv = node_to_i915(m->private);
 	struct drm_device *dev = &dev_priv->drm;
 	struct drm_file *file;
+	int ret;
 
-	int ret = mutex_lock_interruptible(&dev->struct_mutex);
+	mutex_lock(&dev->filelist_mutex);
+	ret = mutex_lock_interruptible(&dev->struct_mutex);
 	if (ret)
-		return ret;
+		goto out_unlock;
+
 	intel_runtime_pm_get(dev_priv);
 
 	if (INTEL_GEN(dev_priv) >= 8)
@@ -2281,7 +2284,6 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
 	else if (INTEL_GEN(dev_priv) >= 6)
 		gen6_ppgtt_info(m, dev_priv);
 
-	mutex_lock(&dev->filelist_mutex);
 	list_for_each_entry_reverse(file, &dev->filelist, lhead) {
 		struct drm_i915_file_private *file_priv = file->driver_priv;
 		struct task_struct *task;
@@ -2289,19 +2291,19 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
 		task = get_pid_task(file->pid, PIDTYPE_PID);
 		if (!task) {
 			ret = -ESRCH;
-			goto out_unlock;
+			goto out_rpm;
 		}
 		seq_printf(m, "\nproc: %s\n", task->comm);
 		put_task_struct(task);
 		idr_for_each(&file_priv->context_idr, per_file_ctx,
 			     (void *)(unsigned long)m);
 	}
-out_unlock:
-	mutex_unlock(&dev->filelist_mutex);
 
+out_rpm:
 	intel_runtime_pm_put(dev_priv);
 	mutex_unlock(&dev->struct_mutex);
-
+out_unlock:
+	mutex_unlock(&dev->filelist_mutex);
 	return ret;
 }