From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=pass header.oldest-pass=0 smtp.remote-ip=40.107.200.35 ARC-Seal: i=2; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=pass; t=1793011585; b=qBnZMc76a1qkqB8EeAU64rIVmgKO6JgUkat8zkEtS/ikufZMo3ajmLByi9AwwRDn+2E3 tYTSyHHhutfpmCttRS3yRDmWN0P8SQFk/ciAO+LsxpfLwkegRZwyHbcDJldWQ3qOV1NXQ BhxSXqb6lIqoIHPIVwZWQsRGPPk4U7Lp30H69h3a8BGhmHq8VEwlvUJu+V9YfstPJALnP wLbTszw4xkD9V3s2kMKm94Us3f466pez7NAPCQRAzD9QcAjVXfIYkKcLNIoeI3yz4g58r IFFGkxBX4bBhDvSd63EPEVuxIpBdAGrSR07sLWiWykUAb1xj4OqTtCH0LHVmCi1Q9xA== ARC-Message-Signature: i=2; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793011585; h=DMARC-Filter:DKIM-Signature:Received:Received:Message-ID:Date: User-Agent:Subject:To:Cc:References:Content-Language:From:In-Reply-To: Content-Type:Content-Transfer-Encoding:X-ClientProxiedBy:MIME-Version: X-MS-PublicTrafficType:X-MS-TrafficTypeDiagnostic: X-MS-Office365-Filtering-Correlation-Id:X-MS-Exchange-SenderADCheck: X-MS-Exchange-AntiSpam-Relay:X-Microsoft-Antispam: X-Microsoft-Antispam-Message-Info:X-Forefront-Antispam-Report: X-MS-Exchange-AntiSpam-MessageData-ChunkCount: X-MS-Exchange-AntiSpam-MessageData-0:X-OriginatorOrg: X-MS-Exchange-CrossTenant-Network-Message-Id: X-MS-Exchange-CrossTenant-AuthSource:X-MS-Exchange-CrossTenant-AuthAs: X-MS-Exchange-CrossTenant-OriginalArrivalTime: X-MS-Exchange-CrossTenant-FromEntityHeader: X-MS-Exchange-CrossTenant-Id:X-MS-Exchange-CrossTenant-MailboxType: X-MS-Exchange-CrossTenant-UserPrincipalName: X-MS-Exchange-Transport-CrossTenantHeadersStamped; bh=J5UtFe3iQ3jVOlSuFiyJIq6ocLzdpm+IgQ5PhdGzYZk=; b=XpWe6x63B++IaxKJZpqwDvs7YtKU118h26K6oI8x+C3xz/iVlgm/w3q/Ph9yFIQ1EUz+ 1Gy/8IYyd8dfxjz+Wb7HqG0eCWN/kLF7ZCi0iY9H59roHi+2XJp4Ngtn+FkSeOwDFHmdy 6EFSLL1niEX7VVLP/7ynJI2LH5urFXGUFaPqQUOwPZdh5PYyiEL3iuhTwcO5evuZrWRyp CQWpxTfoD3Ew4WtdxIlm57upgEk/XssM/WWNweIpqDwADHy/ldOPMz6UgOGs+WzHURD3T vhc+CYSuiAwCPEYZnavJlUZMH84OEk+wYCpw7vmS6E1fn+VKD0DsPC7U1EjAIKNpC6Q== ARC-Authentication-Results: i=2; smtp.subspace.kernel.org; dmarc=pass header.from=amd.com; spf=fail smtp.mailfrom=amd.com; dkim=pass header.d=amd.com header.i=@amd.com header.a=rsa-sha256 header.s=selector1 header.b=jCMrZAE9; arc=pass header.oldest-pass=0 smtp.remote-ip=40.107.200.35 Received: from CH5PR02CU005.outbound.protection.outlook.com (mail-northcentralusazon11012035.outbound.protection.outlook.com [40.107.200.35]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (secp384r1) server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 63996606A31A for ; Mon, 26 Oct 2026 10:46:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 63996606A31A Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amd.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=amd.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=amd.com header.i=@amd.com header.a=rsa-sha256 header.s=selector1 header.b=jCMrZAE9 ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=WrRKLo9rxwaYQMzGgEfxyqXKalRgSr38QONs3MpDlEkdTXFz4KeIVpGvE8aRCdLyY+O4MOhNYvPZcQqPN01m3RpVlb0FCCTQhjRU170g3udAzJk4ofq1Ncs+NHFpg5XRZiJansOPgs98n9FbuIOkRvtmttmYKJ/g46NsCtTIea5uFnPd3JTQFKFdwhY8h+hmMHFkHPSMlg1TMIPEJbag/X0smYOPJmJ8IzPteUhKo006qQgz9mULuYRLKnAaqVycyXsmTnPQdvFtiTI3rPrkOwqpiDLb9xJKU0SGtbrZgF6r2b4scQyBJUvEpcBse0QEow52ChTEwe59eommdw/W/w== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=J5UtFe3iQ3jVOlSuFiyJIq6ocLzdpm+IgQ5PhdGzYZk=; b=l8y3+RsR7CUYPcdkCi1WHGiwGdWXzCseRg4fREKFwrxJZ+ZUvU9fl/ZL4ApoyBAhAf4fmfox4xt5dECCZbZLXxd0R3gOhbc4NS9pL14EfDOX/a6MxQQPCDpx6hpAHFa2VXGziAEhXknVeBVrTY1Q+NvDLP71ztODq8B5frbE+59ExyrBAi9vs3TvGPRlERooNnOmyo8qG+ZVNtuBuKs8PXVBay1+w/fc8vsDWbGNlgFSLtI1DTGeL38U+3YjYuL+dlOMbmvkkicuZWftzPhlKMvIHklYTnwuZUHc97OIlUnBsRd39ceQDPnbnqgntaK0Z0jkM9fvxyBfYF0+RpIo+A== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=amd.com; dmarc=pass action=none header.from=amd.com; dkim=pass header.d=amd.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amd.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=J5UtFe3iQ3jVOlSuFiyJIq6ocLzdpm+IgQ5PhdGzYZk=; b=jCMrZAE9oyXiLD+EjXiXa1zLI9QYCU4ZZHABQAsnmeLMGwC0pN7EkP/fAZS86xMLx2AP0lbtzXs17gz12yYCdV37fUXBo8S8zHVTMlF2Iza9z37EDsMgenJXR4g1+CWpxpLZf8wLsiryS5PDFlN2U1wIB1GgPuocWa/tJTZcuU4= Authentication-Results: mx.microsoft.com 1; dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) by DS4PR12MB9635.namprd12.prod.outlook.com (2603:10b6:8:281::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.546.21; Mon, 26 Oct 2026 10:46:23 +0000 Received: from PH7PR12MB5685.namprd12.prod.outlook.com ([fe80::ce69:cfae:774d:a65c]) by PH7PR12MB5685.namprd12.prod.outlook.com ([fe80::ce69:cfae:774d:a65c%3]) with mapi id 15.21.0546.021; Mon, 26 Oct 2026 10:46:22 +0000 Message-ID: <1a76be29-7c68-47c9-9c76-443a409db151@amd.com> Date: Mon, 26 Oct 2026 11:46:18 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] dma-buf/dma-fence: Document why signaled-bit is set early To: Philipp Stanner , Sumit Semwal , Tvrtko Ursulin , Danilo Krummrich Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20261023100047.2589846-3-phasta@kernel.org> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <20261023100047.2589846-3-phasta@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FRYP281CA0006.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10::16) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|DS4PR12MB9635:EE_ X-MS-Office365-Filtering-Correlation-Id: d8fda0dc-e038-43d1-3c8c-08df334e6046 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|366016|1800799024|11063799006|56012099006|261021121427099003|261022075727099003|261009223027099003|10067099003|18002099003|22082099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:PH7PR12MB5685.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(23010399003)(376014)(366016)(1800799024)(11063799006)(56012099006)(261021121427099003)(261022075727099003)(261009223027099003)(10067099003)(18002099003)(22082099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?VWwyczgyZk9UdVByQk5xekJKWDNYVk9KbEJsWWxQRjFBbnRzbkxKNXIxR3A4?= =?utf-8?B?USsvQ3dNOEdzQSthZ1hEd3gzWnRBYkNTUGRiZkg1d2ZpNWFJc0liNXE0U2J4?= =?utf-8?B?Q0JiVWRneTdWSnhPYzZibUxzNUlKNG9JZXJVZGRkZUVGejlIdTJrQlFYeks4?= =?utf-8?B?bkUvUDBsK0lCRDRpWnVZNGVjbGNWNk04QmN2VlcrdE5iamIrTCtFZzBLMHJy?= =?utf-8?B?ZzQrekg3Z3RlcnRkQU1GWFRpZDNWUWcycjBJL0FyQ21veGNaWFVKeW1VaDBZ?= =?utf-8?B?MUNyZkZGYzVpa2NVcWpXSzEzM3lkck9qU1FNbStEZ0VJWmN3QXdYTUxWU1Nq?= =?utf-8?B?UlRWeVNveXM2bStUQ3dKTGQ1Vk5vOTdyajdrS05zekxpdHVKL1hxSmRWcUd3?= =?utf-8?B?WFVXQmRpRjhnQkthaXdSODd0Ym44ZjQzM3VBdXZERFRrZFN3dDh1MnBzQVdq?= =?utf-8?B?bFk3bmdEakhEUXdIWHZ3VFF2NUxEWWt6dW1QK2xtL1FzSFhmQnRUd0FtYm9Y?= =?utf-8?B?OWJRZWVidkJFUWxKWENaM1ZKekhlWEkwUUVzek5IbmxtQ2s1dzdYbGtMcW9m?= =?utf-8?B?czRrQTZDSEJIRnVRK3NFK3JrSUJqYUtlZ3pnR1NTdUp2WGJ1N1g1a0NkeGhR?= =?utf-8?B?bFRtRVZURDhBL0JYYWwyMkpzbDVxckh5TzRQMEVSdEwxVlAyY2FlZXE5UWNu?= =?utf-8?B?UklvVnpHN0pXVGw3bVNHbGNxd0RPTHR2U3F2T0RaM0RIY3ZJSkZsL1BrSjIx?= =?utf-8?B?Z0hDaTVpS1h6bytBYTdoeERkd1pqNTd4MnE2dDIxVHlTZ3JnaUtpZFlvUkhI?= =?utf-8?B?VUZOM0lHRWJyT3M0amUyamZCcUUzQlViYzVwZkVLSnJja2lOL0J4Qk83V2RE?= =?utf-8?B?dUtBSmhRUEppclp0cGozaHV4RFVKb25QKzRlZkxFaFJBZjc0aFB2MURMMVht?= =?utf-8?B?ejRrbThJS2ZmanpkQ1dJNFZsOXMzb3NZelNTV016YVBYVkNTandGRVZ1TlVq?= =?utf-8?B?WlUyRXFITEJyOENZMDlsdjJzRkFRcU9ONEhPaXVQRVdScnkyc200WVFuTGg2?= =?utf-8?B?QWxxcHdmeXhVUVc4bjRhUE9wdHJjTzBiT2Ixaks4QUFKRVN3VVRjMTVleFdI?= =?utf-8?B?Qy82dW91OE1DQUFHczl0M3B4ekd2b1lPRGdiR0ZlSHp3SVRUaWR4dzE2dUdw?= =?utf-8?B?SlpGdThrSDd2NGNyWkF3aUhLdGVkdWdzNWlEd3JEdm9QbGt6dk5xSHAreHYy?= =?utf-8?B?cEE0RTdqTlNvNVlPc3lxb2JZeDJ0aVplZGNWTVNPcXN1SE90U2R4RlpwdlJE?= =?utf-8?B?MEpmL3lpbTVIUGVwTk5jODBpL0VlVTZzNWpYNFBDVmtVRHNyS01yTk82K1Rr?= =?utf-8?B?enNKZEVoV1dwdVB4SFZJcjRaN2dPeko1QXY0dnRJKzVJa1ZkWTU5VUxmWlpM?= =?utf-8?B?L2JFWnhKZTdqWlcrU3g2L20zR0JoZ2NDNjRqczVtWGs0YnNjbmtmWWNSWmNk?= =?utf-8?B?VVliMTA4ZERsM2ZncXVwaS9wUE9IZW5lNEZpQzF4YWZKT0phNUJtSS9UejdV?= =?utf-8?B?Zm55VTFndTlrTnlWbFJZTFpKTTFFWitJZW5wRE5GOFpWcmFjYTIzS0czemdU?= =?utf-8?B?L1diYzZYTEtWcXM3RGVpa2NKVFRrK205dklha1hoYUEzbThCNVZEQVJ6Ykt4?= =?utf-8?B?aUhYVzRUQ2VyeVlkVzU4OWhRMlU2ZkJMZjVJRVZaQjY3Sm96bDdCVmt3cXlC?= =?utf-8?B?MkFzSlNYb3pVakw0a2dkUUNTSmFmcVFmbHFXWm4rZmIyaUwvb2xod3hzMWdZ?= =?utf-8?B?VGpTTWxqczcwU0dRSmN0Nm9KODZlbkNYU2FmSFFtZDQxYWcrWmRFNGV5UHRU?= =?utf-8?B?eWJlUmN1bFdXYmNUdTlXN0VFbXVDSHdrbUUrdW14N0pJcGtBcXZDVmVBL0xD?= =?utf-8?B?TVpxcW5zMkFhL0dXRWdUOU1SODNQaGlvK1pySUU0U01xQThVV0NENkNPQXEz?= =?utf-8?B?K3dMaG9KV1l4UEl4eWRWVVZtbzRkeHhleXV6MGplT2hCczZCQkkyeFliaEJs?= =?utf-8?B?VmF5Rld5eUVqVUpweDNtUHFKVllxL1J6UHY1ZmFGb1Y2aWJZVEN1R08vSHhO?= =?utf-8?B?bVZaNGo5eWVjOG16YXhyd2RwOFp5eEpVaGpFQ1Y4MW5iOE4reGUwL090Mmtj?= =?utf-8?B?NFY1Z0dDL2lWcXRoTW5yUUorbDdJSmxFUHZxdHA2cks3R2h1czhSWk9UdkNm?= =?utf-8?B?L0VrS1FINjVZVWRhZHFUeFJiM3daTTBIRVBHTllWTWJVL2huTk54aVcvQWZk?= =?utf-8?Q?dd3yUzm5NKfCG1NL07?= X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-Network-Message-Id: d8fda0dc-e038-43d1-3c8c-08df334e6046 X-MS-Exchange-CrossTenant-AuthSource: PH7PR12MB5685.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Oct 2026 10:46:22.6780 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: q++CI7Qh+wD8k2rluoCsmCHN2tYRnekbJ7LYPVnZ79ZNQwrMpXJy0kvRajBwOCrU X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS4PR12MB9635 On 10/23/26 12:00, Philipp Stanner wrote: > There have been repeated discussions about why the signaled-bit is set > before a fence is actually done signaling. Repeatedly, people try to > change that. > > As long as the bit can be read locklessly, for example by > dma_fence_is_signaled(), the bit cannot be set at any other place. > > Document that to reduce further noise about the topic. > > Signed-off-by: Philipp Stanner Reviewed-by: Christian König > --- > I thought that if people repeatedly try to make a change, and it > repeatedly gets rejected, then we can all save ourselves time and work > by quickly documenting it :-) > > This way, we also remember in 5 years *why* something was or wasn't > done. > > Philipp > --- > drivers/dma-buf/dma-fence.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c > index bd58688b81a7..a71e18a9e346 100644 > --- a/drivers/dma-buf/dma-fence.c > +++ b/drivers/dma-buf/dma-fence.c > @@ -359,6 +359,16 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence, > > dma_fence_assert_held(fence); > > + /* > + * It would be better to set the bit after the callbacks ran so that the > + * fast-path-check in dma_fence_is_signaled() cannot return true while > + * they are running. > + * > + * That is, however, impossible because many parties now check for the > + * signaled-state of a fence in their callbacks (or in the work_items > + * kicked by them). So we have to keep setting it here as long as the > + * signaled-bit is read locklessly by anyone. > + */ > if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > &fence->flags))) > return; > > base-commit: cf5d8e390e2ac2cfa24a50645efdbb9f0eebe65f