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linux/include/linux/preempt.h
Thomas Gleixner 66630058e5 sched/rt: Provide migrate_disable/enable() inlines 
Code which solely needs to prevent migration of a task uses
preempt_disable()/enable() pairs. This is the only reliable way to do so
as setting the task affinity to a single CPU can be undone by a
setaffinity operation from a different task/process.

RT provides a seperate migrate_disable/enable() mechanism which does not
disable preemption to achieve the semantic requirements of a (almost) fully
preemptible kernel.

As it is unclear from looking at a given code path whether the intention is
to disable preemption or migration, introduce migrate_disable/enable()
inline functions which can be used to annotate code which merely needs to
disable migration. Map them to preempt_disable/enable() for now. The RT
substitution will be provided later.

Code which is annotated that way documents that it has no requirement to
protect against reentrancy of a preempting task. Either this is not
required at all or the call sites are already serialized by other means.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/878slclv1u.fsf@nanos.tec.linutronix.de
2020-02-20 21:17:24 +01:00

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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: 0x00100000
* PREEMPT_NEED_RESCHED: 0x80000000
*/
#define PREEMPT_BITS 8
#define SOFTIRQ_BITS 8
#define HARDIRQ_BITS 4
#define NMI_BITS 1
#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>
#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
#define softirq_count() (preempt_count() & SOFTIRQ_MASK)
#define irq_count() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK \
| NMI_MASK))
/*
* Are we doing bottom half or hardware interrupt processing?
*
* in_irq() - We're in (hard) IRQ context
* in_softirq() - We have BH disabled, or are processing softirqs
* in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
* in_serving_softirq() - We're in softirq context
* in_nmi() - We're in NMI context
* in_task() - We're in task context
*
* Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really
* should not be used in new code.
*/
#define in_irq() (hardirq_count())
#define in_softirq() (softirq_count())
#define in_interrupt() (irq_count())
#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
#define in_nmi() (preempt_count() & NMI_MASK)
#define in_task() (!(preempt_count() & \
(NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
/*
* 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()
*/
#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
/*
* 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(&notifier->link);
notifier->ops = ops;
}
#endif
/**
* migrate_disable - Prevent migration of the current task
*
* Maps to preempt_disable() which also disables preemption. Use
* migrate_disable() to annotate that the intent is to prevent migration,
* but not necessarily preemption.
*
* Can be invoked nested like preempt_disable() and needs the corresponding
* number of migrate_enable() invocations.
*/
static __always_inline void migrate_disable(void)
{
preempt_disable();
}
/**
* migrate_enable - Allow migration of the current task
*
* Counterpart to migrate_disable().
*
* As migrate_disable() can be invoked nested, only the outermost invocation
* reenables migration.
*
* Currently mapped to preempt_enable().
*/
static __always_inline void migrate_enable(void)
{
preempt_enable();
}
#endif /* __LINUX_PREEMPT_H */
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