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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Dec 2025 13:26:19.8924 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: f6eda159-bc0e-4f37-3b53-08de3ca6b30f 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: AM3PEPF00009B9E.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PA6PR08MB10594 > On Tue Dec 16, 2025 at 12:01 PM UTC, Yeoreum Yun wrote: > >> 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)*. > > But this phrasing sounds like there are other non-preemptive contexts > that it _does_ support. I would definitely read this as implying that > plain old preempt_disable() is OK. I don't understand what those "few > other strict contexts" are, nor why the stop_machine() context is > included in them. I think this phrasing seems to consider non-preeptive case for the priority or schedule policy but still make me confused too. But What I worth to say the stop_machine() -- exactly the callback context (stopper thread context) by stop_machine() is the same for raw_spin_lock() case where explictly disable preemption by calling preempt_disable(). The reason why raw_spin_lock() context couldn't call the GFP_ATOMIC since it explicitly disable preemption by calling preempt_disable(). stop_machine() callback context -- stopper thread's context is also the same. when it calls the callback by stopper (see cpu_stopper_thread()): ... preempt_count_inc(); ret = fn(arg); ... preempt_count_dec(); ... preemption is explicitly disabled like raw_spin_lock() So it seems to include in "few strict non-preemptive context". -- Sincerely, Yeoreum Yun