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completion uses a wait_queue_head_t to enqueue waiters.
wait_queue_head_t contains a spinlock_t to protect the list of waiters
which excludes it from being used in truly atomic context on a PREEMPT_RT
enabled kernel.
The spinlock in the wait queue head cannot be replaced by a raw_spinlock
because:
- wait queues can have custom wakeup callbacks, which acquire other
spinlock_t locks and have potentially long execution times
- wake_up() walks an unbounded number of list entries during the wake up
and may wake an unbounded number of waiters.
For simplicity and performance reasons complete() should be usable on
PREEMPT_RT enabled kernels.
completions do not use custom wakeup callbacks and are usually single
waiter, except for a few corner cases.
Replace the wait queue in the completion with a simple wait queue (swait),
which uses a raw_spinlock_t for protecting the waiter list and therefore is
safe to use inside truly atomic regions on PREEMPT_RT.
There is no semantical or functional change:
- completions use the exclusive wait mode which is what swait provides
- complete() wakes one exclusive waiter
- complete_all() wakes all waiters while holding the lock which protects
the wait queue against newly incoming waiters. The conversion to swait
preserves this behaviour.
complete_all() might cause unbound latencies with a large number of waiters
being woken at once, but most complete_all() usage sites are either in
testing or initialization code or have only a really small number of
concurrent waiters which for now does not cause a latency problem. Keep it
simple for now.
The fixup of the warning check in the USB gadget driver is just a straight
forward conversion of the lockless waiter check from one waitqueue type to
the other.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200321113242.317954042@linutronix.de
122 lines
4.1 KiB
C
122 lines
4.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_COMPLETION_H
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#define __LINUX_COMPLETION_H
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/*
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* (C) Copyright 2001 Linus Torvalds
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*
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* Atomic wait-for-completion handler data structures.
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* See kernel/sched/completion.c for details.
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*/
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#include <linux/swait.h>
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/*
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* struct completion - structure used to maintain state for a "completion"
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*
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* This is the opaque structure used to maintain the state for a "completion".
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* Completions currently use a FIFO to queue threads that have to wait for
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* the "completion" event.
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*
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* See also: complete(), wait_for_completion() (and friends _timeout,
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* _interruptible, _interruptible_timeout, and _killable), init_completion(),
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* reinit_completion(), and macros DECLARE_COMPLETION(),
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* DECLARE_COMPLETION_ONSTACK().
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*/
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struct completion {
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unsigned int done;
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struct swait_queue_head wait;
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};
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#define init_completion_map(x, m) __init_completion(x)
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#define init_completion(x) __init_completion(x)
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static inline void complete_acquire(struct completion *x) {}
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static inline void complete_release(struct completion *x) {}
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#define COMPLETION_INITIALIZER(work) \
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{ 0, __SWAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
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#define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \
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(*({ init_completion_map(&(work), &(map)); &(work); }))
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#define COMPLETION_INITIALIZER_ONSTACK(work) \
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(*({ init_completion(&work); &work; }))
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/**
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* DECLARE_COMPLETION - declare and initialize a completion structure
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* @work: identifier for the completion structure
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*
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* This macro declares and initializes a completion structure. Generally used
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* for static declarations. You should use the _ONSTACK variant for automatic
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* variables.
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*/
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#define DECLARE_COMPLETION(work) \
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struct completion work = COMPLETION_INITIALIZER(work)
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/*
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* Lockdep needs to run a non-constant initializer for on-stack
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* completions - so we use the _ONSTACK() variant for those that
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* are on the kernel stack:
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*/
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/**
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* DECLARE_COMPLETION_ONSTACK - declare and initialize a completion structure
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* @work: identifier for the completion structure
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*
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* This macro declares and initializes a completion structure on the kernel
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* stack.
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*/
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#ifdef CONFIG_LOCKDEP
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# define DECLARE_COMPLETION_ONSTACK(work) \
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struct completion work = COMPLETION_INITIALIZER_ONSTACK(work)
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# define DECLARE_COMPLETION_ONSTACK_MAP(work, map) \
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struct completion work = COMPLETION_INITIALIZER_ONSTACK_MAP(work, map)
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#else
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# define DECLARE_COMPLETION_ONSTACK(work) DECLARE_COMPLETION(work)
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# define DECLARE_COMPLETION_ONSTACK_MAP(work, map) DECLARE_COMPLETION(work)
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#endif
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/**
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* init_completion - Initialize a dynamically allocated completion
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* @x: pointer to completion structure that is to be initialized
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*
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* This inline function will initialize a dynamically created completion
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* structure.
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*/
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static inline void __init_completion(struct completion *x)
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{
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x->done = 0;
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init_swait_queue_head(&x->wait);
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}
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/**
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* reinit_completion - reinitialize a completion structure
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* @x: pointer to completion structure that is to be reinitialized
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*
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* This inline function should be used to reinitialize a completion structure so it can
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* be reused. This is especially important after complete_all() is used.
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*/
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static inline void reinit_completion(struct completion *x)
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{
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x->done = 0;
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}
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extern void wait_for_completion(struct completion *);
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extern void wait_for_completion_io(struct completion *);
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extern int wait_for_completion_interruptible(struct completion *x);
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extern int wait_for_completion_killable(struct completion *x);
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extern unsigned long wait_for_completion_timeout(struct completion *x,
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unsigned long timeout);
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extern unsigned long wait_for_completion_io_timeout(struct completion *x,
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unsigned long timeout);
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extern long wait_for_completion_interruptible_timeout(
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struct completion *x, unsigned long timeout);
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extern long wait_for_completion_killable_timeout(
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struct completion *x, unsigned long timeout);
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extern bool try_wait_for_completion(struct completion *x);
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extern bool completion_done(struct completion *x);
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extern void complete(struct completion *);
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extern void complete_all(struct completion *);
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#endif
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