Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sun, 11 Sep 2022 23:13:53 +0200
From:   =?utf-8?Q?Micha=C5=82?= Winiarski <michal.winiarski@intel.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>,
        <intel-gfx@lists.freedesktop.org>, David Airlie <airlied@linux.ie>,
        "Daniel Vetter" <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Simon Ser <contact@emersion.fr>
Subject: Re: [PATCH v4 1/3] drm: Use XArray instead of IDR for minors
Message-ID: <20220911211353.kuv3jnwkgo2m5arw@nostramo>
References: <20220906201629.419160-1-michal.winiarski@intel.com>
 <20220906201629.419160-2-michal.winiarski@intel.com>
 <Yxe1YDcVfqbjgT6a@casper.infradead.org>
Content-Type: text/plain; charset="utf-8"
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <Yxe1YDcVfqbjgT6a@casper.infradead.org>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?SkpGaUw5eXN1WnY2a25XbCtKL0Nrb1huYUlsVkN6TGdMbkgzSEdYaFBGY1lV?=
 =?utf-8?B?VTEwZkVUWitZbmU4akFrUjUwWXFmV2J2ajdEMlV5bFdtcDdLWjBIUWlmSVcw?=
 =?utf-8?B?TlFkRzFEakdZMklsZmFObHhabVJrYmIvWGJkNmxBUmhGeWt0MS9mRTM0VWNu?=
 =?utf-8?B?a1AxQ0VDNW9WMm94aTZ0MzcxaXkxWjI1RWtrNVMxM1AzMGVQZXdEK2F3dUdF?=
 =?utf-8?B?K0RCaEMvYVk2bCtFZkdiZ3lUZDlNa1pGYW9MSEVtYlpHWjl0MGFGOTNnMkhC?=
 =?utf-8?B?ajErS3ZmeVdoQkhZbURGQTl3Y3J3RXh6WHF0VjdwVXVzQ3ZaOU5DcWVRTk9U?=
 =?utf-8?B?Y21EM2k3U0lXNXkxS3RQQXAwSmo3a05QZDk2NkRkMXJJZzZCUWxCdzNzN1ZF?=
 =?utf-8?B?UkNIM1htenk0aE9IK3h4SmJBRDdsTTVRbUNSL09uc2t2Ykw2SGMyeEhSMm80?=
 =?utf-8?B?VjBDM2JPQUVMQy9wMDRLakYrZC9rVDZQdEtwSUF0MCtWWWZGWmhranVxc3BD?=
 =?utf-8?B?QmFTdGFpKzVPcCtZK0xybkRXek1EZXB3clptc0c5MDIrRVJFWDl5WU5KQ0pU?=
 =?utf-8?B?eERmdThnVkwyL1dVNU5VUHA2bkNLdTgwUnp6bmFUNng3S3dqQnJSQjNCZ0hD?=
 =?utf-8?B?T09ublBUa281S3d5VmduOHNXeFVEVHRiMEJZK3BXb25hb2xCVitpT29qNFdC?=
 =?utf-8?B?dlF3dTdWRytxNk9tckJXQkJ5MWZaRi9aUWZyMVcyKy9hbzY1RzdYa0Rudnl4?=
 =?utf-8?B?V3dMR2EzM1NOUlo0YlJlRCtrbTkraWtCTU8yYm5yeTlKSzdRNlBKdC9QOU55?=
 =?utf-8?B?c3VjNzJNRTNrdDh3emFnM2FiY0haQjVMVjJwS1ozSkhlOFQwUU5kcnc5WEhO?=
 =?utf-8?B?UGRTTUxQaFQreEFweHUzM3NHSlF3UkV2T3JMT0FJZ3NrdzN5NU8vV1FZN1d0?=
 =?utf-8?B?NVJIK2d3cENhV2pFTU5sUUNMQnFFSXo3SDIvOHFYNUtQTldLaUFvc2hIdTRn?=
 =?utf-8?B?cE1reURDWG85TWlZTUVlMnp4ZHVqQWF3d2dGanNBMUNpNUhFbUJLZEx0WTAw?=
 =?utf-8?B?Y0p6Y0kvcXowTzhzTGE3NWIvY1hYOHA2cm1xb0t4Q25SVmVtYjN1ODFhenhy?=
 =?utf-8?B?YmU4VWZldDM4eHpHSkZ2UGtLRm9IbjE4Nmg0S1hrQlRYYnNTL3FmZ1lYV1JE?=
 =?utf-8?B?UXE0SjQyK3FGWUhTSHVZS3BQSm5nVXNaSDdvTlFkQWw0aGFzdFVndVh2NGRG?=
 =?utf-8?B?RWRWS3ZFTTIxK2xtcXNWY1oyMU10RW1tV3dxM1VrSkMrSVFZV0ZRZ2dpRE5s?=
 =?utf-8?B?VmdUQmgxUHNrNzhCeGVQa3F1MEJuUUpCdTVLWVNpWmRWcElUSnllWUZYTERB?=
 =?utf-8?B?QnBUSVVSWDc3RlBSWGxlQWRTMjBZMjhPenMzMmhSNFFKZHBKWG1obDZkRVVv?=
 =?utf-8?B?K1JLUjRrV01HcXpQZ1VGNVdIdWJ1Zmg4b2NaOWM3c0RySE9aeS9KSXdIdU1W?=
 =?utf-8?B?SjRJSjBMVEtYQS9BMVNKQlZiOHVrYkY1dDM3UFh0UFJOZUdMd083RlpWMXBo?=
 =?utf-8?B?WkhoS3BRY2xwVytGQmVvTkFqK3V5aVgvL3FGRGpCZzEwakt6V0FST0Z1T00w?=
 =?utf-8?B?VHR0eXlYV2U5TVJxWTNXRURWVUZOWXp3VWR0b1BiT0tHZnVRRTFGRjZRWUpZ?=
 =?utf-8?B?MUUvbXRISThuRmJ5VFBRS1hnRGoyV1VneXBhZFJPeTFleFVRSEkxUGFZOWdo?=
 =?utf-8?B?Uk5oNWh5OWxoZUFxcFBmMERyT2pFbTEyQVhBeTNVUjRnamQwTnZEMGFTN0RI?=
 =?utf-8?B?MUlGOW5KY3FoeEx1WlZEZmJiZUI3Mm1RTVUrVFYzMit1THVRMjJyWFN4WDdX?=
 =?utf-8?B?NnhnQjV4eDJFbFhvWlRMaTh6aUxobXhHRG5DZFk3SWJlMit1KzVGem5ZSHBT?=
 =?utf-8?B?NEJMNHZKdUNZS3lRWmhzbXRKb0JPUUVUdDJzc2JHVWhJQzVFZDVUNGpmUmpL?=
 =?utf-8?B?NkpyaXZWTGNGK0VTeE1IejFwa1ZKbmIzcVFWSXJLMHB4K091T1J2QjFvcXVH?=
 =?utf-8?B?TTU5cHFQSkJWLyt4TGdEMVJVdFVKYUtjWXQxdFhFMVhlRllBQXNTQVR3c0xE?=
 =?utf-8?B?TTFBWHZkOHVETzRPVWVuQUJTejRWTXBtcCtkUmdVWVJhWmRGZ0hkRnFDZDBO?=
 =?utf-8?B?eUE9PQ==?=
X-MS-Exchange-CrossTenant-Network-Message-Id: cde79016-0703-478b-98d3-08da943a8c18
X-MS-Exchange-CrossTenant-AuthSource: DM4PR11MB5373.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Sep 2022 21:14:00.7345
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: SfAzcesRM22ucBxYcNDVpvdAVHcSdWCYS3MHhirnYiZYDLgxgn+0CnXiodim9kps2cdWIrdA/20/21CN9bI5PijaQp9sfV0LKsNNK+zzt7c=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM4PR11MB6065
X-OriginatorOrg: intel.com
X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,
        SPF_HELO_NONE,SPF_NONE,T_SCC_BODY_TEXT_LINE autolearn=ham
        autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Sep 06, 2022 at 10:02:24PM +0100, Matthew Wilcox wrote:
> On Tue, Sep 06, 2022 at 10:16:27PM +0200, Michał Winiarski wrote:
> > IDR is deprecated, and since XArray manages its own state with internal
> > locking, it simplifies the locking on DRM side.
> > Additionally, don't use the IRQ-safe variant, since operating on drm
> > minor is not done in IRQ context.
> > 
> > Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
> > Suggested-by: Matthew Wilcox <willy@infradead.org>
> 
> I have a few questions, but I like where you're going.
> 
> > @@ -98,21 +98,18 @@ static struct drm_minor **drm_minor_get_slot(struct drm_device *dev,
> >  static void drm_minor_alloc_release(struct drm_device *dev, void *data)
> >  {
> >  	struct drm_minor *minor = data;
> > -	unsigned long flags;
> >  
> >  	WARN_ON(dev != minor->dev);
> >  
> >  	put_device(minor->kdev);
> >  
> > -	spin_lock_irqsave(&drm_minor_lock, flags);
> > -	idr_remove(&drm_minors_idr, minor->index);
> > -	spin_unlock_irqrestore(&drm_minor_lock, flags);
> > +	xa_release(&drm_minors_xa, minor->index);
> 
> Has it definitely been unused at this point?  I would think that
> xa_erase() (an unconditional store) would be the correct function to
> call.

Yes, unless there's a programmers error somewhere - I'll replace it though.

> 
> > @@ -122,20 +119,12 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
> >  	minor->type = type;
> >  	minor->dev = dev;
> >  
> > -	idr_preload(GFP_KERNEL);
> > -	spin_lock_irqsave(&drm_minor_lock, flags);
> > -	r = idr_alloc(&drm_minors_idr,
> > -		      NULL,
> > -		      64 * type,
> > -		      64 * (type + 1),
> > -		      GFP_NOWAIT);
> > -	spin_unlock_irqrestore(&drm_minor_lock, flags);
> > -	idr_preload_end();
> > -
> > +	r = xa_alloc(&drm_minors_xa, &id, NULL,
> > +		     XA_LIMIT(64 * type, 64 * (type + 1) - 1), GFP_KERNEL);
> >  	if (r < 0)
> >  		return r;
> >  
> > -	minor->index = r;
> > +	minor->index = id;
> 
> Wouldn't it be better to call:
> 
> 	r = xa_alloc(&drm_minors_xa, &minor->index, NULL,
> 			XA_LIMIT(64 * type, 64 * (type + 1) - 1), GFP_KERNEL);
> 
> I might also prefer a little syntactic sugar like:
> 
> #define DRM_MINOR_LIMIT(type)	XA_LIMIT(64 * (type), 64 * (type) + 63)
> 
> but that's definitely a matter of taste.

Sure.

> 
> > @@ -172,9 +161,12 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
> >  		goto err_debugfs;
> >  
> >  	/* replace NULL with @minor so lookups will succeed from now on */
> > -	spin_lock_irqsave(&drm_minor_lock, flags);
> > -	idr_replace(&drm_minors_idr, minor, minor->index);
> > -	spin_unlock_irqrestore(&drm_minor_lock, flags);
> > +	entry = xa_store(&drm_minors_xa, minor->index, &minor, GFP_KERNEL);
> > +	if (xa_is_err(entry)) {
> > +		ret = xa_err(entry);
> > +		goto err_debugfs;
> > +	}
> > +	WARN_ON(entry);
> 
> Might be better as an xa_cmpxchg()?

Ack.

> 
> > @@ -187,16 +179,13 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
> >  static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
> >  {
> >  	struct drm_minor *minor;
> > -	unsigned long flags;
> >  
> >  	minor = *drm_minor_get_slot(dev, type);
> >  	if (!minor || !device_is_registered(minor->kdev))
> >  		return;
> >  
> >  	/* replace @minor with NULL so lookups will fail from now on */
> > -	spin_lock_irqsave(&drm_minor_lock, flags);
> > -	idr_replace(&drm_minors_idr, NULL, minor->index);
> > -	spin_unlock_irqrestore(&drm_minor_lock, flags);
> > +	xa_erase(&drm_minors_xa, minor->index);
> 
> This isn't an exact replacement, but I'm not sure whether that makes a
> difference.  xa_erase() allows allocation of this ID again while
> idr_replace() means that lookups return NULL, but the ID remains in
> use.  The equivalent of idr_replace() is:
> 	xa_store(&drm_minors_xa, minor->index, NULL, GFP_KERNEL);

It does makes a difference, I'll change it to the equivalent.

> 
> > @@ -215,13 +204,10 @@ static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
> >  struct drm_minor *drm_minor_acquire(unsigned int minor_id)
> >  {
> >  	struct drm_minor *minor;
> > -	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&drm_minor_lock, flags);
> > -	minor = idr_find(&drm_minors_idr, minor_id);
> > +	minor = xa_load(&drm_minors_xa, minor_id);
> >  	if (minor)
> >  		drm_dev_get(minor->dev);
> > -	spin_unlock_irqrestore(&drm_minor_lock, flags);
> 
> This is also not an exact equivalent as the dev_drm_get() is now outside
> the lock.  Does that matter in this case?  I don't know the code well
> enough to say.  If you want it to be equivalent, then:
> 
> 	xa_lock(&drm_minors_xa);
> 	minor = xa_load(&drm_minors_xa, minor_id);
> 	if (minor)
> 		drm_dev_get(minor->dev);
> 	xa_unlock(&drm_minors_xa);
> 
> would be the code to use.

Again, it does matter, I'll change it.

> 
> > @@ -1037,7 +1023,7 @@ static void drm_core_exit(void)
> >  	unregister_chrdev(DRM_MAJOR, "drm");
> >  	debugfs_remove(drm_debugfs_root);
> >  	drm_sysfs_destroy();
> > -	idr_destroy(&drm_minors_idr);
> > +	xa_destroy(&drm_minors_xa);
> 
> I don't know if this is the right call.  xa_destroy() is the exact
> replacement, but if the xarray should already be empty (and it should,
> right?) then asserting the xa_empty() is true may be the better call
> to make.

Yeah - I'll replace it with WARN_ON.

> 
> 
> Phew, that was a lot of comments/questions.  I hope that was useful!

Very useful indeed, thanks!

-Michał