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- /* SPDX-License-Identifier: GPL-2.0 */
- #ifndef __LINUX_PREEMPT_H
- #define __LINUX_PREEMPT_H
- /*
- * include/linux/preempt.h - macros for accessing and manipulating
- * preempt_count (used for kernel preemption, interrupt count, etc.)
- */
- #include <linux/linkage.h>
- #include <linux/list.h>
- /*
- * We put the hardirq and softirq counter into the preemption
- * counter. The bitmask has the following meaning:
- *
- * - bits 0-7 are the preemption count (max preemption depth: 256)
- * - bits 8-15 are the softirq count (max # of softirqs: 256)
- *
- * The hardirq count could in theory be the same as the number of
- * interrupts in the system, but we run all interrupt handlers with
- * interrupts disabled, so we cannot have nesting interrupts. Though
- * there are a few palaeontologic drivers which reenable interrupts in
- * the handler, so we need more than one bit here.
- *
- * PREEMPT_MASK: 0x000000ff
- * SOFTIRQ_MASK: 0x0000ff00
- * HARDIRQ_MASK: 0x000f0000
- * NMI_MASK: 0x00f00000
- * PREEMPT_NEED_RESCHED: 0x80000000
- */
- #define PREEMPT_BITS 8
- #define SOFTIRQ_BITS 8
- #define HARDIRQ_BITS 4
- #define NMI_BITS 4
- #define PREEMPT_SHIFT 0
- #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS)
- #define HARDIRQ_SHIFT (SOFTIRQ_SHIFT + SOFTIRQ_BITS)
- #define NMI_SHIFT (HARDIRQ_SHIFT + HARDIRQ_BITS)
- #define __IRQ_MASK(x) ((1UL << (x))-1)
- #define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
- #define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
- #define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
- #define NMI_MASK (__IRQ_MASK(NMI_BITS) << NMI_SHIFT)
- #define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)
- #define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT)
- #define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
- #define NMI_OFFSET (1UL << NMI_SHIFT)
- #define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)
- #define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
- /*
- * Disable preemption until the scheduler is running -- use an unconditional
- * value so that it also works on !PREEMPT_COUNT kernels.
- *
- * Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().
- */
- #define INIT_PREEMPT_COUNT PREEMPT_OFFSET
- /*
- * Initial preempt_count value; reflects the preempt_count schedule invariant
- * which states that during context switches:
- *
- * preempt_count() == 2*PREEMPT_DISABLE_OFFSET
- *
- * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
- * Note: See finish_task_switch().
- */
- #define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
- /* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
- #include <asm/preempt.h>
- /**
- * interrupt_context_level - return interrupt context level
- *
- * Returns the current interrupt context level.
- * 0 - normal context
- * 1 - softirq context
- * 2 - hardirq context
- * 3 - NMI context
- */
- static __always_inline unsigned char interrupt_context_level(void)
- {
- unsigned long pc = preempt_count();
- unsigned char level = 0;
- level += !!(pc & (NMI_MASK));
- level += !!(pc & (NMI_MASK | HARDIRQ_MASK));
- level += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
- return level;
- }
- /*
- * These macro definitions avoid redundant invocations of preempt_count()
- * because such invocations would result in redundant loads given that
- * preempt_count() is commonly implemented with READ_ONCE().
- */
- #define nmi_count() (preempt_count() & NMI_MASK)
- #define hardirq_count() (preempt_count() & HARDIRQ_MASK)
- #ifdef CONFIG_PREEMPT_RT
- # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK)
- # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
- #else
- # define softirq_count() (preempt_count() & SOFTIRQ_MASK)
- # define irq_count() (preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK))
- #endif
- /*
- * Macros to retrieve the current execution context:
- *
- * in_nmi() - We're in NMI context
- * in_hardirq() - We're in hard IRQ context
- * in_serving_softirq() - We're in softirq context
- * in_task() - We're in task context
- */
- #define in_nmi() (nmi_count())
- #define in_hardirq() (hardirq_count())
- #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
- #ifdef CONFIG_PREEMPT_RT
- # define in_task() (!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq()))
- #else
- # define in_task() (!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
- #endif
- /*
- * The following macros are deprecated and should not be used in new code:
- * in_irq() - Obsolete version of in_hardirq()
- * in_softirq() - We have BH disabled, or are processing softirqs
- * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
- */
- #define in_irq() (hardirq_count())
- #define in_softirq() (softirq_count())
- #define in_interrupt() (irq_count())
- /*
- * The preempt_count offset after preempt_disable();
- */
- #if defined(CONFIG_PREEMPT_COUNT)
- # define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET
- #else
- # define PREEMPT_DISABLE_OFFSET 0
- #endif
- /*
- * The preempt_count offset after spin_lock()
- */
- #if !defined(CONFIG_PREEMPT_RT)
- #define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
- #else
- /* Locks on RT do not disable preemption */
- #define PREEMPT_LOCK_OFFSET 0
- #endif
- /*
- * The preempt_count offset needed for things like:
- *
- * spin_lock_bh()
- *
- * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
- * softirqs, such that unlock sequences of:
- *
- * spin_unlock();
- * local_bh_enable();
- *
- * Work as expected.
- */
- #define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
- /*
- * Are we running in atomic context? WARNING: this macro cannot
- * always detect atomic context; in particular, it cannot know about
- * held spinlocks in non-preemptible kernels. Thus it should not be
- * used in the general case to determine whether sleeping is possible.
- * Do not use in_atomic() in driver code.
- */
- #define in_atomic() (preempt_count() != 0)
- /*
- * Check whether we were atomic before we did preempt_disable():
- * (used by the scheduler)
- */
- #define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
- #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
- extern void preempt_count_add(int val);
- extern void preempt_count_sub(int val);
- #define preempt_count_dec_and_test() \
- ({ preempt_count_sub(1); should_resched(0); })
- #else
- #define preempt_count_add(val) __preempt_count_add(val)
- #define preempt_count_sub(val) __preempt_count_sub(val)
- #define preempt_count_dec_and_test() __preempt_count_dec_and_test()
- #endif
- #define __preempt_count_inc() __preempt_count_add(1)
- #define __preempt_count_dec() __preempt_count_sub(1)
- #define preempt_count_inc() preempt_count_add(1)
- #define preempt_count_dec() preempt_count_sub(1)
- #ifdef CONFIG_PREEMPT_COUNT
- #define preempt_disable() \
- do { \
- preempt_count_inc(); \
- barrier(); \
- } while (0)
- #define sched_preempt_enable_no_resched() \
- do { \
- barrier(); \
- preempt_count_dec(); \
- } while (0)
- #define preempt_enable_no_resched() sched_preempt_enable_no_resched()
- #define preemptible() (preempt_count() == 0 && !irqs_disabled())
- #ifdef CONFIG_PREEMPTION
- #define preempt_enable() \
- do { \
- barrier(); \
- if (unlikely(preempt_count_dec_and_test())) \
- __preempt_schedule(); \
- } while (0)
- #define preempt_enable_notrace() \
- do { \
- barrier(); \
- if (unlikely(__preempt_count_dec_and_test())) \
- __preempt_schedule_notrace(); \
- } while (0)
- #define preempt_check_resched() \
- do { \
- if (should_resched(0)) \
- __preempt_schedule(); \
- } while (0)
- #else /* !CONFIG_PREEMPTION */
- #define preempt_enable() \
- do { \
- barrier(); \
- preempt_count_dec(); \
- } while (0)
- #define preempt_enable_notrace() \
- do { \
- barrier(); \
- __preempt_count_dec(); \
- } while (0)
- #define preempt_check_resched() do { } while (0)
- #endif /* CONFIG_PREEMPTION */
- #define preempt_disable_notrace() \
- do { \
- __preempt_count_inc(); \
- barrier(); \
- } while (0)
- #define preempt_enable_no_resched_notrace() \
- do { \
- barrier(); \
- __preempt_count_dec(); \
- } while (0)
- #else /* !CONFIG_PREEMPT_COUNT */
- /*
- * Even if we don't have any preemption, we need preempt disable/enable
- * to be barriers, so that we don't have things like get_user/put_user
- * that can cause faults and scheduling migrate into our preempt-protected
- * region.
- */
- #define preempt_disable() barrier()
- #define sched_preempt_enable_no_resched() barrier()
- #define preempt_enable_no_resched() barrier()
- #define preempt_enable() barrier()
- #define preempt_check_resched() do { } while (0)
- #define preempt_disable_notrace() barrier()
- #define preempt_enable_no_resched_notrace() barrier()
- #define preempt_enable_notrace() barrier()
- #define preemptible() 0
- #endif /* CONFIG_PREEMPT_COUNT */
- #ifdef MODULE
- /*
- * Modules have no business playing preemption tricks.
- */
- #undef sched_preempt_enable_no_resched
- #undef preempt_enable_no_resched
- #undef preempt_enable_no_resched_notrace
- #undef preempt_check_resched
- #endif
- #define preempt_set_need_resched() \
- do { \
- set_preempt_need_resched(); \
- } while (0)
- #define preempt_fold_need_resched() \
- do { \
- if (tif_need_resched()) \
- set_preempt_need_resched(); \
- } while (0)
- #ifdef CONFIG_PREEMPT_NOTIFIERS
- struct preempt_notifier;
- /**
- * preempt_ops - notifiers called when a task is preempted and rescheduled
- * @sched_in: we're about to be rescheduled:
- * notifier: struct preempt_notifier for the task being scheduled
- * cpu: cpu we're scheduled on
- * @sched_out: we've just been preempted
- * notifier: struct preempt_notifier for the task being preempted
- * next: the task that's kicking us out
- *
- * Please note that sched_in and out are called under different
- * contexts. sched_out is called with rq lock held and irq disabled
- * while sched_in is called without rq lock and irq enabled. This
- * difference is intentional and depended upon by its users.
- */
- struct preempt_ops {
- void (*sched_in)(struct preempt_notifier *notifier, int cpu);
- void (*sched_out)(struct preempt_notifier *notifier,
- struct task_struct *next);
- };
- /**
- * preempt_notifier - key for installing preemption notifiers
- * @link: internal use
- * @ops: defines the notifier functions to be called
- *
- * Usually used in conjunction with container_of().
- */
- struct preempt_notifier {
- struct hlist_node link;
- struct preempt_ops *ops;
- };
- void preempt_notifier_inc(void);
- void preempt_notifier_dec(void);
- void preempt_notifier_register(struct preempt_notifier *notifier);
- void preempt_notifier_unregister(struct preempt_notifier *notifier);
- static inline void preempt_notifier_init(struct preempt_notifier *notifier,
- struct preempt_ops *ops)
- {
- INIT_HLIST_NODE(¬ifier->link);
- notifier->ops = ops;
- }
- #endif
- #ifdef CONFIG_SMP
- /*
- * Migrate-Disable and why it is undesired.
- *
- * When a preempted task becomes elegible to run under the ideal model (IOW it
- * becomes one of the M highest priority tasks), it might still have to wait
- * for the preemptee's migrate_disable() section to complete. Thereby suffering
- * a reduction in bandwidth in the exact duration of the migrate_disable()
- * section.
- *
- * Per this argument, the change from preempt_disable() to migrate_disable()
- * gets us:
- *
- * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
- * it would have had to wait for the lower priority task.
- *
- * - a lower priority tasks; which under preempt_disable() could've instantly
- * migrated away when another CPU becomes available, is now constrained
- * by the ability to push the higher priority task away, which might itself be
- * in a migrate_disable() section, reducing it's available bandwidth.
- *
- * IOW it trades latency / moves the interference term, but it stays in the
- * system, and as long as it remains unbounded, the system is not fully
- * deterministic.
- *
- *
- * The reason we have it anyway.
- *
- * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
- * number of primitives into becoming preemptible, they would also allow
- * migration. This turns out to break a bunch of per-cpu usage. To this end,
- * all these primitives employ migirate_disable() to restore this implicit
- * assumption.
- *
- * This is a 'temporary' work-around at best. The correct solution is getting
- * rid of the above assumptions and reworking the code to employ explicit
- * per-cpu locking or short preempt-disable regions.
- *
- * The end goal must be to get rid of migrate_disable(), alternatively we need
- * a schedulability theory that does not depend on abritrary migration.
- *
- *
- * Notes on the implementation.
- *
- * The implementation is particularly tricky since existing code patterns
- * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
- * This means that it cannot use cpus_read_lock() to serialize against hotplug,
- * nor can it easily migrate itself into a pending affinity mask change on
- * migrate_enable().
- *
- *
- * Note: even non-work-conserving schedulers like semi-partitioned depends on
- * migration, so migrate_disable() is not only a problem for
- * work-conserving schedulers.
- *
- */
- extern void migrate_disable(void);
- extern void migrate_enable(void);
- #else
- static inline void migrate_disable(void) { }
- static inline void migrate_enable(void) { }
- #endif /* CONFIG_SMP */
- /**
- * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
- *
- * Use for code which requires preemption protection inside a critical
- * section which has preemption disabled implicitly on non-PREEMPT_RT
- * enabled kernels, by e.g.:
- * - holding a spinlock/rwlock
- * - soft interrupt context
- * - regular interrupt handlers
- *
- * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
- * interrupt context and regular interrupt handlers are preemptible and
- * only prevent migration. preempt_disable_nested() ensures that preemption
- * is disabled for cases which require CPU local serialization even on
- * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
- *
- * The use cases are code sequences which are not serialized by a
- * particular lock instance, e.g.:
- * - seqcount write side critical sections where the seqcount is not
- * associated to a particular lock and therefore the automatic
- * protection mechanism does not work. This prevents a live lock
- * against a preempting high priority reader.
- * - RMW per CPU variable updates like vmstat.
- */
- /* Macro to avoid header recursion hell vs. lockdep */
- #define preempt_disable_nested() \
- do { \
- if (IS_ENABLED(CONFIG_PREEMPT_RT)) \
- preempt_disable(); \
- else \
- lockdep_assert_preemption_disabled(); \
- } while (0)
- /**
- * preempt_enable_nested - Undo the effect of preempt_disable_nested()
- */
- static __always_inline void preempt_enable_nested(void)
- {
- if (IS_ENABLED(CONFIG_PREEMPT_RT))
- preempt_enable();
- }
- #endif /* __LINUX_PREEMPT_H */
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