completion: Use simple wait queues
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
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@ -1703,7 +1703,7 @@ static void ffs_data_put(struct ffs_data *ffs)
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pr_info("%s(): freeing\n", __func__);
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ffs_data_clear(ffs);
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BUG_ON(waitqueue_active(&ffs->ev.waitq) ||
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waitqueue_active(&ffs->ep0req_completion.wait) ||
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swait_active(&ffs->ep0req_completion.wait) ||
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waitqueue_active(&ffs->wait));
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destroy_workqueue(ffs->io_completion_wq);
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kfree(ffs->dev_name);
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@ -9,7 +9,7 @@
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* See kernel/sched/completion.c for details.
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*/
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#include <linux/wait.h>
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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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@ -25,7 +25,7 @@
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*/
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struct completion {
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unsigned int done;
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wait_queue_head_t wait;
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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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@ -34,7 +34,7 @@ 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, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
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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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@ -85,7 +85,7 @@ static inline void complete_release(struct completion *x) {}
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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_waitqueue_head(&x->wait);
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init_swait_queue_head(&x->wait);
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}
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/**
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@ -29,12 +29,12 @@ void complete(struct completion *x)
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{
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unsigned long flags;
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spin_lock_irqsave(&x->wait.lock, flags);
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raw_spin_lock_irqsave(&x->wait.lock, flags);
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if (x->done != UINT_MAX)
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x->done++;
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__wake_up_locked(&x->wait, TASK_NORMAL, 1);
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spin_unlock_irqrestore(&x->wait.lock, flags);
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swake_up_locked(&x->wait);
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raw_spin_unlock_irqrestore(&x->wait.lock, flags);
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}
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EXPORT_SYMBOL(complete);
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@ -58,10 +58,12 @@ void complete_all(struct completion *x)
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{
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unsigned long flags;
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spin_lock_irqsave(&x->wait.lock, flags);
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WARN_ON(irqs_disabled());
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raw_spin_lock_irqsave(&x->wait.lock, flags);
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x->done = UINT_MAX;
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__wake_up_locked(&x->wait, TASK_NORMAL, 0);
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spin_unlock_irqrestore(&x->wait.lock, flags);
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swake_up_all_locked(&x->wait);
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raw_spin_unlock_irqrestore(&x->wait.lock, flags);
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}
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EXPORT_SYMBOL(complete_all);
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@ -70,20 +72,20 @@ do_wait_for_common(struct completion *x,
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long (*action)(long), long timeout, int state)
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{
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if (!x->done) {
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DECLARE_WAITQUEUE(wait, current);
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DECLARE_SWAITQUEUE(wait);
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__add_wait_queue_entry_tail_exclusive(&x->wait, &wait);
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do {
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if (signal_pending_state(state, current)) {
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timeout = -ERESTARTSYS;
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break;
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}
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__prepare_to_swait(&x->wait, &wait);
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__set_current_state(state);
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spin_unlock_irq(&x->wait.lock);
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raw_spin_unlock_irq(&x->wait.lock);
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timeout = action(timeout);
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spin_lock_irq(&x->wait.lock);
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raw_spin_lock_irq(&x->wait.lock);
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} while (!x->done && timeout);
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__remove_wait_queue(&x->wait, &wait);
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__finish_swait(&x->wait, &wait);
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if (!x->done)
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return timeout;
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}
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@ -100,9 +102,9 @@ __wait_for_common(struct completion *x,
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complete_acquire(x);
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spin_lock_irq(&x->wait.lock);
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raw_spin_lock_irq(&x->wait.lock);
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timeout = do_wait_for_common(x, action, timeout, state);
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spin_unlock_irq(&x->wait.lock);
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raw_spin_unlock_irq(&x->wait.lock);
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complete_release(x);
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@ -291,12 +293,12 @@ bool try_wait_for_completion(struct completion *x)
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if (!READ_ONCE(x->done))
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return false;
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spin_lock_irqsave(&x->wait.lock, flags);
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raw_spin_lock_irqsave(&x->wait.lock, flags);
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if (!x->done)
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ret = false;
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else if (x->done != UINT_MAX)
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x->done--;
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spin_unlock_irqrestore(&x->wait.lock, flags);
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raw_spin_unlock_irqrestore(&x->wait.lock, flags);
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return ret;
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}
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EXPORT_SYMBOL(try_wait_for_completion);
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@ -322,8 +324,8 @@ bool completion_done(struct completion *x)
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* otherwise we can end up freeing the completion before complete()
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* is done referencing it.
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*/
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spin_lock_irqsave(&x->wait.lock, flags);
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spin_unlock_irqrestore(&x->wait.lock, flags);
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raw_spin_lock_irqsave(&x->wait.lock, flags);
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raw_spin_unlock_irqrestore(&x->wait.lock, flags);
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return true;
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}
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EXPORT_SYMBOL(completion_done);
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