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Tue, 12 Mar 2024 12:48:35 +0000 Date: Tue, 12 Mar 2024 12:48:20 +0000 From: Szabolcs Nagy To: Andrew Pinski , GNU C Library Subject: Re: swapcontext happening on different threads Message-ID: References: Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: LO2P123CA0101.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:139::16) To DB9PR08MB7179.eurprd08.prod.outlook.com (2603:10a6:10:2cc::19) MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: DB9PR08MB7179:EE_|DBAPR08MB5720:EE_|DB1PEPF000509EF:EE_|AS4PR08MB7853:EE_ X-MS-Office365-Filtering-Correlation-Id: cb6996f8-d0cc-4a9d-8331-08dc4292c03c x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0; X-Microsoft-Antispam-Message-Info-Original: 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 X-Forefront-Antispam-Report-Untrusted: CIP:255.255.255.255; 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CTRY:IE; LANG:en; SCL:1; SRV:; IPV:CAL; SFV:NSPM; H:64aa7808-outbound-1.mta.getcheckrecipient.com; PTR:ec2-63-35-35-123.eu-west-1.compute.amazonaws.com; CAT:NONE; SFS:(13230031)(36860700004)(376005)(1800799015)(82310400014); DIR:OUT; SFP:1101; X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Mar 2024 12:48:43.7617 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: cb6996f8-d0cc-4a9d-8331-08dc4292c03c X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d; Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF000509EF.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS4PR08MB7853 X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: libc-alpha-bounces+e=80x24.org@sourceware.org The 03/12/2024 00:24, Andrew Pinski wrote: > Reference https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65712 . > > I am not 100% sure if this is a valid thing to do. > But take the code in the referenced GCC bug report. Currently since > pthread_self is marked as const, if you swap to a different context > and then have that context be swapped on a different thread, you > might/will get the old value of pthread_self in some cases. > > Is this well defined or is this a corner case that was not expected. I > don't see anywhere in the older versions of POSIX that explain what > would happen if you do that. And glibc's documentation does not talk > about this case either. > > It would be useful if the documentation talked about this either way. yeah. my view is that resuming on a different thread is invalid. (tls address or other thread specific state cached on the stack is broken). there are known users who switch between threads via swapcontext, i think they are on their own to ensure things work since they are in undefined land (maybe the compiler could help with appropriate barriers). otoh const pthread_self is theoretically wrong even without swapcontext breaking it: the compiler can cache const func output into a per-process global variable as far as i can tell. (iirc there were bugzilla discussions before that attribute const should not allow exception return or different return values in different threads. btw this affects __errno_location too in glibc)