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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Dec 2025 12:02:14.9735 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 925cc9e4-f389-405d-511e-08de3c9af410 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB3PEPF0000885E.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS8PR08MB8898 > On Tue Dec 16, 2025 at 11:03 AM UTC, Yeoreum Yun wrote: > > Hi Brendan, > > > >> On Mon Dec 15, 2025 at 10:06 AM UTC, Yeoreum Yun wrote: > >> [snip] > >> >> Overall I am feeling a bit uncomfortable about this use of _nolock, but > >> >> I am also feeling pretty ignorant about PREEMPT_RT and also about this > >> >> arm64 code, so I am hesitant to suggest alternatives, I hope someone > >> >> else can offer some input here... > >> > > >> > I understand. However, as I mentioned earlier, > >> > my main intention was to hear opinions specifically about memory contention. > >> > > >> > That said, if there is no memory contention, > >> > I don’t think using the _nolock API is necessarily a bad approach. > >> > >> > >> > In fact, I believe a bigger issue is that, under PREEMPT_RT, > >> > code that uses the regular memory allocation APIs may give users the false impression > >> > that those APIs are “safe to use,” even though they are not. > >> > >> Yeah, I share this concern. I would bet I have written code that's > >> broken under PREEMPT_RT (luckily only in Google's kernel fork). The > >> comment for GFP_ATOMIC says: > >> > >> * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > >> * watermark is applied to allow access to "atomic reserves". > >> * The current implementation doesn't support NMI and few other strict > >> * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT. > >> > >> It kinda sounds like it's supposed to be OK to use GFP_ATOMIC in a > >> normal preempt_disable() context. So do you know exactly why it's > >> invalid to use it in this stop_machine() context here? Maybe we need to > >> update this comment. > > > > In non-PREEMPT_RT configurations, this is fine to use. > > However, in PREEMPT_RT, it should not be used because > > spin_lock becomes a sleepable lock backed by an rt-mutex. > > > > From Documentation/locking/locktypes.rst: > > > > The fact that PREEMPT_RT changes the lock category of spinlock_t and > > rwlock_t from spinning to sleeping. > > > > As you know, all locks related to memory allocation > > (e.g., zone_lock, PCP locks, etc.) use spin_lock, > > which becomes sleepable under PREEMPT_RT. > > > > The callback of stop_machine() is executed in a preemption-disabled context > > (see cpu_stopper_thread()). In this context, if it fails to acquire a spinlock > > during memory allocation, > > the task would be able to go to sleep while preemption is disabled, > > which is an obviously problematic situation. > > But this is what I mean, doesn't this sound like the GFP_ATOMIC comment > I quoted is wrong (or at least, it implies things which are wrong)? The > comment refers specifically to raw_spin_lock() and "strict > non-preemptive contexts". Which sounds like it is being written with > PREEMPT_RT in mind. But that doesn't really match what you've said. No. I think the comment of GFP_ATOMIC is right. It definitely said: The current implementation *doesn't support* NMI and few other strict *non-preemptive contexts (e.g. raw_spin_lock)*. The reason It couldn't be support GFP_ATOMIC in raw_spin_lock() context in PREEMPT_RT since critical section protected by raw_spin_lock() is non-preemptive (preemption disabled). This is the same reason "GFP_ATOMIC" cannot be used in the stop_machine(). -- Sincerely, Yeoreum Yun