1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Context tracking: Probe on high level context boundaries such as kernel,
4 * userspace, guest or idle.
5 *
6 * This is used by RCU to remove its dependency on the timer tick while a CPU
7 * runs in idle, userspace or guest mode.
8 *
9 * User/guest tracking started by Frederic Weisbecker:
10 *
11 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker
12 *
13 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
14 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
15 *
16 * RCU extended quiescent state bits imported from kernel/rcu/tree.c
17 * where the relevant authorship may be found.
18 */
19
20 #include <linux/context_tracking.h>
21 #include <linux/rcupdate.h>
22 #include <linux/sched.h>
23 #include <linux/hardirq.h>
24 #include <linux/export.h>
25 #include <linux/kprobes.h>
26 #include <trace/events/rcu.h>
27
28
29 DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
30 #ifdef CONFIG_CONTEXT_TRACKING_IDLE
31 .dynticks_nesting = 1,
32 .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
33 #endif
34 .state = ATOMIC_INIT(RCU_DYNTICKS_IDX),
35 };
36 EXPORT_SYMBOL_GPL(context_tracking);
37
38 #ifdef CONFIG_CONTEXT_TRACKING_IDLE
39 #define TPS(x) tracepoint_string(x)
40
41 /* Record the current task on dyntick-idle entry. */
rcu_dynticks_task_enter(void)42 static __always_inline void rcu_dynticks_task_enter(void)
43 {
44 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
45 WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
46 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
47 }
48
49 /* Record no current task on dyntick-idle exit. */
rcu_dynticks_task_exit(void)50 static __always_inline void rcu_dynticks_task_exit(void)
51 {
52 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
53 WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
54 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
55 }
56
57 /* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
rcu_dynticks_task_trace_enter(void)58 static __always_inline void rcu_dynticks_task_trace_enter(void)
59 {
60 #ifdef CONFIG_TASKS_TRACE_RCU
61 if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
62 current->trc_reader_special.b.need_mb = true;
63 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
64 }
65
66 /* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
rcu_dynticks_task_trace_exit(void)67 static __always_inline void rcu_dynticks_task_trace_exit(void)
68 {
69 #ifdef CONFIG_TASKS_TRACE_RCU
70 if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
71 current->trc_reader_special.b.need_mb = false;
72 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
73 }
74
75 /*
76 * Record entry into an extended quiescent state. This is only to be
77 * called when not already in an extended quiescent state, that is,
78 * RCU is watching prior to the call to this function and is no longer
79 * watching upon return.
80 */
ct_kernel_exit_state(int offset)81 static noinstr void ct_kernel_exit_state(int offset)
82 {
83 int seq;
84
85 /*
86 * CPUs seeing atomic_add_return() must see prior RCU read-side
87 * critical sections, and we also must force ordering with the
88 * next idle sojourn.
89 */
90 rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
91 seq = ct_state_inc(offset);
92 // RCU is no longer watching. Better be in extended quiescent state!
93 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & RCU_DYNTICKS_IDX));
94 }
95
96 /*
97 * Record exit from an extended quiescent state. This is only to be
98 * called from an extended quiescent state, that is, RCU is not watching
99 * prior to the call to this function and is watching upon return.
100 */
ct_kernel_enter_state(int offset)101 static noinstr void ct_kernel_enter_state(int offset)
102 {
103 int seq;
104
105 /*
106 * CPUs seeing atomic_add_return() must see prior idle sojourns,
107 * and we also must force ordering with the next RCU read-side
108 * critical section.
109 */
110 seq = ct_state_inc(offset);
111 // RCU is now watching. Better not be in an extended quiescent state!
112 rcu_dynticks_task_trace_exit(); // After ->dynticks update!
113 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICKS_IDX));
114 }
115
116 /*
117 * Enter an RCU extended quiescent state, which can be either the
118 * idle loop or adaptive-tickless usermode execution.
119 *
120 * We crowbar the ->dynticks_nmi_nesting field to zero to allow for
121 * the possibility of usermode upcalls having messed up our count
122 * of interrupt nesting level during the prior busy period.
123 */
ct_kernel_exit(bool user,int offset)124 static void noinstr ct_kernel_exit(bool user, int offset)
125 {
126 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
127
128 WARN_ON_ONCE(ct_dynticks_nmi_nesting() != DYNTICK_IRQ_NONIDLE);
129 WRITE_ONCE(ct->dynticks_nmi_nesting, 0);
130 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
131 ct_dynticks_nesting() == 0);
132 if (ct_dynticks_nesting() != 1) {
133 // RCU will still be watching, so just do accounting and leave.
134 ct->dynticks_nesting--;
135 return;
136 }
137
138 instrumentation_begin();
139 lockdep_assert_irqs_disabled();
140 trace_rcu_dyntick(TPS("Start"), ct_dynticks_nesting(), 0, ct_dynticks());
141 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
142 rcu_preempt_deferred_qs(current);
143
144 // instrumentation for the noinstr ct_kernel_exit_state()
145 instrument_atomic_write(&ct->state, sizeof(ct->state));
146
147 instrumentation_end();
148 WRITE_ONCE(ct->dynticks_nesting, 0); /* Avoid irq-access tearing. */
149 // RCU is watching here ...
150 ct_kernel_exit_state(offset);
151 // ... but is no longer watching here.
152 rcu_dynticks_task_enter();
153 }
154
155 /*
156 * Exit an RCU extended quiescent state, which can be either the
157 * idle loop or adaptive-tickless usermode execution.
158 *
159 * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
160 * allow for the possibility of usermode upcalls messing up our count of
161 * interrupt nesting level during the busy period that is just now starting.
162 */
ct_kernel_enter(bool user,int offset)163 static void noinstr ct_kernel_enter(bool user, int offset)
164 {
165 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
166 long oldval;
167
168 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
169 oldval = ct_dynticks_nesting();
170 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0);
171 if (oldval) {
172 // RCU was already watching, so just do accounting and leave.
173 ct->dynticks_nesting++;
174 return;
175 }
176 rcu_dynticks_task_exit();
177 // RCU is not watching here ...
178 ct_kernel_enter_state(offset);
179 // ... but is watching here.
180 instrumentation_begin();
181
182 // instrumentation for the noinstr ct_kernel_enter_state()
183 instrument_atomic_write(&ct->state, sizeof(ct->state));
184
185 trace_rcu_dyntick(TPS("End"), ct_dynticks_nesting(), 1, ct_dynticks());
186 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
187 WRITE_ONCE(ct->dynticks_nesting, 1);
188 WARN_ON_ONCE(ct_dynticks_nmi_nesting());
189 WRITE_ONCE(ct->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
190 instrumentation_end();
191 }
192
193 /**
194 * ct_nmi_exit - inform RCU of exit from NMI context
195 *
196 * If we are returning from the outermost NMI handler that interrupted an
197 * RCU-idle period, update ct->state and ct->dynticks_nmi_nesting
198 * to let the RCU grace-period handling know that the CPU is back to
199 * being RCU-idle.
200 *
201 * If you add or remove a call to ct_nmi_exit(), be sure to test
202 * with CONFIG_RCU_EQS_DEBUG=y.
203 */
ct_nmi_exit(void)204 void noinstr ct_nmi_exit(void)
205 {
206 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
207
208 instrumentation_begin();
209 /*
210 * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
211 * (We are exiting an NMI handler, so RCU better be paying attention
212 * to us!)
213 */
214 WARN_ON_ONCE(ct_dynticks_nmi_nesting() <= 0);
215 WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());
216
217 /*
218 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
219 * leave it in non-RCU-idle state.
220 */
221 if (ct_dynticks_nmi_nesting() != 1) {
222 trace_rcu_dyntick(TPS("--="), ct_dynticks_nmi_nesting(), ct_dynticks_nmi_nesting() - 2,
223 ct_dynticks());
224 WRITE_ONCE(ct->dynticks_nmi_nesting, /* No store tearing. */
225 ct_dynticks_nmi_nesting() - 2);
226 instrumentation_end();
227 return;
228 }
229
230 /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
231 trace_rcu_dyntick(TPS("Startirq"), ct_dynticks_nmi_nesting(), 0, ct_dynticks());
232 WRITE_ONCE(ct->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
233
234 // instrumentation for the noinstr ct_kernel_exit_state()
235 instrument_atomic_write(&ct->state, sizeof(ct->state));
236 instrumentation_end();
237
238 // RCU is watching here ...
239 ct_kernel_exit_state(RCU_DYNTICKS_IDX);
240 // ... but is no longer watching here.
241
242 if (!in_nmi())
243 rcu_dynticks_task_enter();
244 }
245
246 /**
247 * ct_nmi_enter - inform RCU of entry to NMI context
248 *
249 * If the CPU was idle from RCU's viewpoint, update ct->state and
250 * ct->dynticks_nmi_nesting to let the RCU grace-period handling know
251 * that the CPU is active. This implementation permits nested NMIs, as
252 * long as the nesting level does not overflow an int. (You will probably
253 * run out of stack space first.)
254 *
255 * If you add or remove a call to ct_nmi_enter(), be sure to test
256 * with CONFIG_RCU_EQS_DEBUG=y.
257 */
ct_nmi_enter(void)258 void noinstr ct_nmi_enter(void)
259 {
260 long incby = 2;
261 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
262
263 /* Complain about underflow. */
264 WARN_ON_ONCE(ct_dynticks_nmi_nesting() < 0);
265
266 /*
267 * If idle from RCU viewpoint, atomically increment ->dynticks
268 * to mark non-idle and increment ->dynticks_nmi_nesting by one.
269 * Otherwise, increment ->dynticks_nmi_nesting by two. This means
270 * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
271 * to be in the outermost NMI handler that interrupted an RCU-idle
272 * period (observation due to Andy Lutomirski).
273 */
274 if (rcu_dynticks_curr_cpu_in_eqs()) {
275
276 if (!in_nmi())
277 rcu_dynticks_task_exit();
278
279 // RCU is not watching here ...
280 ct_kernel_enter_state(RCU_DYNTICKS_IDX);
281 // ... but is watching here.
282
283 instrumentation_begin();
284 // instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
285 instrument_atomic_read(&ct->state, sizeof(ct->state));
286 // instrumentation for the noinstr ct_kernel_enter_state()
287 instrument_atomic_write(&ct->state, sizeof(ct->state));
288
289 incby = 1;
290 } else if (!in_nmi()) {
291 instrumentation_begin();
292 rcu_irq_enter_check_tick();
293 } else {
294 instrumentation_begin();
295 }
296
297 trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
298 ct_dynticks_nmi_nesting(),
299 ct_dynticks_nmi_nesting() + incby, ct_dynticks());
300 instrumentation_end();
301 WRITE_ONCE(ct->dynticks_nmi_nesting, /* Prevent store tearing. */
302 ct_dynticks_nmi_nesting() + incby);
303 barrier();
304 }
305
306 /**
307 * ct_idle_enter - inform RCU that current CPU is entering idle
308 *
309 * Enter idle mode, in other words, -leave- the mode in which RCU
310 * read-side critical sections can occur. (Though RCU read-side
311 * critical sections can occur in irq handlers in idle, a possibility
312 * handled by irq_enter() and irq_exit().)
313 *
314 * If you add or remove a call to ct_idle_enter(), be sure to test with
315 * CONFIG_RCU_EQS_DEBUG=y.
316 */
ct_idle_enter(void)317 void noinstr ct_idle_enter(void)
318 {
319 WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
320 ct_kernel_exit(false, RCU_DYNTICKS_IDX + CONTEXT_IDLE);
321 }
322 EXPORT_SYMBOL_GPL(ct_idle_enter);
323
324 /**
325 * ct_idle_exit - inform RCU that current CPU is leaving idle
326 *
327 * Exit idle mode, in other words, -enter- the mode in which RCU
328 * read-side critical sections can occur.
329 *
330 * If you add or remove a call to ct_idle_exit(), be sure to test with
331 * CONFIG_RCU_EQS_DEBUG=y.
332 */
ct_idle_exit(void)333 void noinstr ct_idle_exit(void)
334 {
335 unsigned long flags;
336
337 raw_local_irq_save(flags);
338 ct_kernel_enter(false, RCU_DYNTICKS_IDX - CONTEXT_IDLE);
339 raw_local_irq_restore(flags);
340 }
341 EXPORT_SYMBOL_GPL(ct_idle_exit);
342
343 /**
344 * ct_irq_enter - inform RCU that current CPU is entering irq away from idle
345 *
346 * Enter an interrupt handler, which might possibly result in exiting
347 * idle mode, in other words, entering the mode in which read-side critical
348 * sections can occur. The caller must have disabled interrupts.
349 *
350 * Note that the Linux kernel is fully capable of entering an interrupt
351 * handler that it never exits, for example when doing upcalls to user mode!
352 * This code assumes that the idle loop never does upcalls to user mode.
353 * If your architecture's idle loop does do upcalls to user mode (or does
354 * anything else that results in unbalanced calls to the irq_enter() and
355 * irq_exit() functions), RCU will give you what you deserve, good and hard.
356 * But very infrequently and irreproducibly.
357 *
358 * Use things like work queues to work around this limitation.
359 *
360 * You have been warned.
361 *
362 * If you add or remove a call to ct_irq_enter(), be sure to test with
363 * CONFIG_RCU_EQS_DEBUG=y.
364 */
ct_irq_enter(void)365 noinstr void ct_irq_enter(void)
366 {
367 lockdep_assert_irqs_disabled();
368 ct_nmi_enter();
369 }
370
371 /**
372 * ct_irq_exit - inform RCU that current CPU is exiting irq towards idle
373 *
374 * Exit from an interrupt handler, which might possibly result in entering
375 * idle mode, in other words, leaving the mode in which read-side critical
376 * sections can occur. The caller must have disabled interrupts.
377 *
378 * This code assumes that the idle loop never does anything that might
379 * result in unbalanced calls to irq_enter() and irq_exit(). If your
380 * architecture's idle loop violates this assumption, RCU will give you what
381 * you deserve, good and hard. But very infrequently and irreproducibly.
382 *
383 * Use things like work queues to work around this limitation.
384 *
385 * You have been warned.
386 *
387 * If you add or remove a call to ct_irq_exit(), be sure to test with
388 * CONFIG_RCU_EQS_DEBUG=y.
389 */
ct_irq_exit(void)390 noinstr void ct_irq_exit(void)
391 {
392 lockdep_assert_irqs_disabled();
393 ct_nmi_exit();
394 }
395
396 /*
397 * Wrapper for ct_irq_enter() where interrupts are enabled.
398 *
399 * If you add or remove a call to ct_irq_enter_irqson(), be sure to test
400 * with CONFIG_RCU_EQS_DEBUG=y.
401 */
ct_irq_enter_irqson(void)402 void ct_irq_enter_irqson(void)
403 {
404 unsigned long flags;
405
406 local_irq_save(flags);
407 ct_irq_enter();
408 local_irq_restore(flags);
409 }
410
411 /*
412 * Wrapper for ct_irq_exit() where interrupts are enabled.
413 *
414 * If you add or remove a call to ct_irq_exit_irqson(), be sure to test
415 * with CONFIG_RCU_EQS_DEBUG=y.
416 */
ct_irq_exit_irqson(void)417 void ct_irq_exit_irqson(void)
418 {
419 unsigned long flags;
420
421 local_irq_save(flags);
422 ct_irq_exit();
423 local_irq_restore(flags);
424 }
425 #else
ct_kernel_exit(bool user,int offset)426 static __always_inline void ct_kernel_exit(bool user, int offset) { }
ct_kernel_enter(bool user,int offset)427 static __always_inline void ct_kernel_enter(bool user, int offset) { }
428 #endif /* #ifdef CONFIG_CONTEXT_TRACKING_IDLE */
429
430 #ifdef CONFIG_CONTEXT_TRACKING_USER
431
432 #define CREATE_TRACE_POINTS
433 #include <trace/events/context_tracking.h>
434
435 DEFINE_STATIC_KEY_FALSE(context_tracking_key);
436 EXPORT_SYMBOL_GPL(context_tracking_key);
437
context_tracking_recursion_enter(void)438 static noinstr bool context_tracking_recursion_enter(void)
439 {
440 int recursion;
441
442 recursion = __this_cpu_inc_return(context_tracking.recursion);
443 if (recursion == 1)
444 return true;
445
446 WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
447 __this_cpu_dec(context_tracking.recursion);
448
449 return false;
450 }
451
context_tracking_recursion_exit(void)452 static __always_inline void context_tracking_recursion_exit(void)
453 {
454 __this_cpu_dec(context_tracking.recursion);
455 }
456
457 /**
458 * __ct_user_enter - Inform the context tracking that the CPU is going
459 * to enter user or guest space mode.
460 *
461 * This function must be called right before we switch from the kernel
462 * to user or guest space, when it's guaranteed the remaining kernel
463 * instructions to execute won't use any RCU read side critical section
464 * because this function sets RCU in extended quiescent state.
465 */
__ct_user_enter(enum ctx_state state)466 void noinstr __ct_user_enter(enum ctx_state state)
467 {
468 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
469 lockdep_assert_irqs_disabled();
470
471 /* Kernel threads aren't supposed to go to userspace */
472 WARN_ON_ONCE(!current->mm);
473
474 if (!context_tracking_recursion_enter())
475 return;
476
477 if (__ct_state() != state) {
478 if (ct->active) {
479 /*
480 * At this stage, only low level arch entry code remains and
481 * then we'll run in userspace. We can assume there won't be
482 * any RCU read-side critical section until the next call to
483 * user_exit() or ct_irq_enter(). Let's remove RCU's dependency
484 * on the tick.
485 */
486 if (state == CONTEXT_USER) {
487 instrumentation_begin();
488 trace_user_enter(0);
489 vtime_user_enter(current);
490 instrumentation_end();
491 }
492 /*
493 * Other than generic entry implementation, we may be past the last
494 * rescheduling opportunity in the entry code. Trigger a self IPI
495 * that will fire and reschedule once we resume in user/guest mode.
496 */
497 rcu_irq_work_resched();
498
499 /*
500 * Enter RCU idle mode right before resuming userspace. No use of RCU
501 * is permitted between this call and rcu_eqs_exit(). This way the
502 * CPU doesn't need to maintain the tick for RCU maintenance purposes
503 * when the CPU runs in userspace.
504 */
505 ct_kernel_exit(true, RCU_DYNTICKS_IDX + state);
506
507 /*
508 * Special case if we only track user <-> kernel transitions for tickless
509 * cputime accounting but we don't support RCU extended quiescent state.
510 * In this we case we don't care about any concurrency/ordering.
511 */
512 if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE))
513 raw_atomic_set(&ct->state, state);
514 } else {
515 /*
516 * Even if context tracking is disabled on this CPU, because it's outside
517 * the full dynticks mask for example, we still have to keep track of the
518 * context transitions and states to prevent inconsistency on those of
519 * other CPUs.
520 * If a task triggers an exception in userspace, sleep on the exception
521 * handler and then migrate to another CPU, that new CPU must know where
522 * the exception returns by the time we call exception_exit().
523 * This information can only be provided by the previous CPU when it called
524 * exception_enter().
525 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
526 * is false because we know that CPU is not tickless.
527 */
528 if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) {
529 /* Tracking for vtime only, no concurrent RCU EQS accounting */
530 raw_atomic_set(&ct->state, state);
531 } else {
532 /*
533 * Tracking for vtime and RCU EQS. Make sure we don't race
534 * with NMIs. OTOH we don't care about ordering here since
535 * RCU only requires RCU_DYNTICKS_IDX increments to be fully
536 * ordered.
537 */
538 raw_atomic_add(state, &ct->state);
539 }
540 }
541 }
542 context_tracking_recursion_exit();
543 }
544 EXPORT_SYMBOL_GPL(__ct_user_enter);
545
546 /*
547 * OBSOLETE:
548 * This function should be noinstr but the below local_irq_restore() is
549 * unsafe because it involves illegal RCU uses through tracing and lockdep.
550 * This is unlikely to be fixed as this function is obsolete. The preferred
551 * way is to call __context_tracking_enter() through user_enter_irqoff()
552 * or context_tracking_guest_enter(). It should be the arch entry code
553 * responsibility to call into context tracking with IRQs disabled.
554 */
ct_user_enter(enum ctx_state state)555 void ct_user_enter(enum ctx_state state)
556 {
557 unsigned long flags;
558
559 /*
560 * Some contexts may involve an exception occuring in an irq,
561 * leading to that nesting:
562 * ct_irq_enter() rcu_eqs_exit(true) rcu_eqs_enter(true) ct_irq_exit()
563 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
564 * helpers are enough to protect RCU uses inside the exception. So
565 * just return immediately if we detect we are in an IRQ.
566 */
567 if (in_interrupt())
568 return;
569
570 local_irq_save(flags);
571 __ct_user_enter(state);
572 local_irq_restore(flags);
573 }
574 NOKPROBE_SYMBOL(ct_user_enter);
575 EXPORT_SYMBOL_GPL(ct_user_enter);
576
577 /**
578 * user_enter_callable() - Unfortunate ASM callable version of user_enter() for
579 * archs that didn't manage to check the context tracking
580 * static key from low level code.
581 *
582 * This OBSOLETE function should be noinstr but it unsafely calls
583 * local_irq_restore(), involving illegal RCU uses through tracing and lockdep.
584 * This is unlikely to be fixed as this function is obsolete. The preferred
585 * way is to call user_enter_irqoff(). It should be the arch entry code
586 * responsibility to call into context tracking with IRQs disabled.
587 */
user_enter_callable(void)588 void user_enter_callable(void)
589 {
590 user_enter();
591 }
592 NOKPROBE_SYMBOL(user_enter_callable);
593
594 /**
595 * __ct_user_exit - Inform the context tracking that the CPU is
596 * exiting user or guest mode and entering the kernel.
597 *
598 * This function must be called after we entered the kernel from user or
599 * guest space before any use of RCU read side critical section. This
600 * potentially include any high level kernel code like syscalls, exceptions,
601 * signal handling, etc...
602 *
603 * This call supports re-entrancy. This way it can be called from any exception
604 * handler without needing to know if we came from userspace or not.
605 */
__ct_user_exit(enum ctx_state state)606 void noinstr __ct_user_exit(enum ctx_state state)
607 {
608 struct context_tracking *ct = this_cpu_ptr(&context_tracking);
609
610 if (!context_tracking_recursion_enter())
611 return;
612
613 if (__ct_state() == state) {
614 if (ct->active) {
615 /*
616 * Exit RCU idle mode while entering the kernel because it can
617 * run a RCU read side critical section anytime.
618 */
619 ct_kernel_enter(true, RCU_DYNTICKS_IDX - state);
620 if (state == CONTEXT_USER) {
621 instrumentation_begin();
622 vtime_user_exit(current);
623 trace_user_exit(0);
624 instrumentation_end();
625 }
626
627 /*
628 * Special case if we only track user <-> kernel transitions for tickless
629 * cputime accounting but we don't support RCU extended quiescent state.
630 * In this we case we don't care about any concurrency/ordering.
631 */
632 if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE))
633 raw_atomic_set(&ct->state, CONTEXT_KERNEL);
634
635 } else {
636 if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) {
637 /* Tracking for vtime only, no concurrent RCU EQS accounting */
638 raw_atomic_set(&ct->state, CONTEXT_KERNEL);
639 } else {
640 /*
641 * Tracking for vtime and RCU EQS. Make sure we don't race
642 * with NMIs. OTOH we don't care about ordering here since
643 * RCU only requires RCU_DYNTICKS_IDX increments to be fully
644 * ordered.
645 */
646 raw_atomic_sub(state, &ct->state);
647 }
648 }
649 }
650 context_tracking_recursion_exit();
651 }
652 EXPORT_SYMBOL_GPL(__ct_user_exit);
653
654 /*
655 * OBSOLETE:
656 * This function should be noinstr but the below local_irq_save() is
657 * unsafe because it involves illegal RCU uses through tracing and lockdep.
658 * This is unlikely to be fixed as this function is obsolete. The preferred
659 * way is to call __context_tracking_exit() through user_exit_irqoff()
660 * or context_tracking_guest_exit(). It should be the arch entry code
661 * responsibility to call into context tracking with IRQs disabled.
662 */
ct_user_exit(enum ctx_state state)663 void ct_user_exit(enum ctx_state state)
664 {
665 unsigned long flags;
666
667 if (in_interrupt())
668 return;
669
670 local_irq_save(flags);
671 __ct_user_exit(state);
672 local_irq_restore(flags);
673 }
674 NOKPROBE_SYMBOL(ct_user_exit);
675 EXPORT_SYMBOL_GPL(ct_user_exit);
676
677 /**
678 * user_exit_callable() - Unfortunate ASM callable version of user_exit() for
679 * archs that didn't manage to check the context tracking
680 * static key from low level code.
681 *
682 * This OBSOLETE function should be noinstr but it unsafely calls local_irq_save(),
683 * involving illegal RCU uses through tracing and lockdep. This is unlikely
684 * to be fixed as this function is obsolete. The preferred way is to call
685 * user_exit_irqoff(). It should be the arch entry code responsibility to
686 * call into context tracking with IRQs disabled.
687 */
user_exit_callable(void)688 void user_exit_callable(void)
689 {
690 user_exit();
691 }
692 NOKPROBE_SYMBOL(user_exit_callable);
693
ct_cpu_track_user(int cpu)694 void __init ct_cpu_track_user(int cpu)
695 {
696 static __initdata bool initialized = false;
697
698 if (!per_cpu(context_tracking.active, cpu)) {
699 per_cpu(context_tracking.active, cpu) = true;
700 static_branch_inc(&context_tracking_key);
701 }
702
703 if (initialized)
704 return;
705
706 #ifdef CONFIG_HAVE_TIF_NOHZ
707 /*
708 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
709 * This assumes that init is the only task at this early boot stage.
710 */
711 set_tsk_thread_flag(&init_task, TIF_NOHZ);
712 #endif
713 WARN_ON_ONCE(!tasklist_empty());
714
715 initialized = true;
716 }
717
718 #ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE
context_tracking_init(void)719 void __init context_tracking_init(void)
720 {
721 int cpu;
722
723 for_each_possible_cpu(cpu)
724 ct_cpu_track_user(cpu);
725 }
726 #endif
727
728 #endif /* #ifdef CONFIG_CONTEXT_TRACKING_USER */
729
