Re: [PATCH] drm/i915: Fix nesting of filelist_mutex vs struct_mutex in i915_ppgtt_info

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 22 Aug 2016, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> [  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]

Do we have a regressing commit reference?

BR,
Jani.

>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_debugfs.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 5193b0447066..c5ef132f8b51 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -2291,10 +2291,13 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
>  	struct drm_device *dev = node->minor->dev;
>  	struct drm_i915_private *dev_priv = to_i915(dev);
>  	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_INFO(dev)->gen >= 8)
> @@ -2302,7 +2305,6 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
>  	else if (INTEL_INFO(dev)->gen >= 6)
>  		gen6_ppgtt_info(m, dev);
>  
> -	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;
> @@ -2310,19 +2312,19 @@ 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;
>  }

-- 
Jani Nikula, Intel Open Source Technology Center
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