Commit a5c6234e authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Peter Zijlstra
Browse files

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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Reviewed-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200321113242.317954042@linutronix.de
parent b3212fe2

drivers/usb/gadget/function/f_fs.c

+1 −1
Original line number Diff line number Diff line
@@ -1703,7 +1703,7 @@ static void ffs_data_put(struct ffs_data *ffs) 
		pr_info("%s(): freeing\n", __func__);

		ffs_data_clear(ffs);

		BUG_ON(waitqueue_active(&ffs->ev.waitq) ||

		       waitqueue_active(&ffs->ep0req_completion.wait) ||

		       swait_active(&ffs->ep0req_completion.wait) ||

		       waitqueue_active(&ffs->wait));

		destroy_workqueue(ffs->io_completion_wq);

		kfree(ffs->dev_name);

include/linux/completion.h

+4 −4
Original line number Diff line number Diff line
@@ -9,7 +9,7 @@ 
 * See kernel/sched/completion.c for details.

 */



#include <linux/wait.h>

#include <linux/swait.h>



/*

 * struct completion - structure used to maintain state for a "completion"
@@ -25,7 +25,7 @@ 
 */

struct completion {

	unsigned int done;

	wait_queue_head_t wait;

	struct swait_queue_head wait;

};



#define init_completion_map(x, m) __init_completion(x)
@@ -34,7 +34,7 @@ static inline void complete_acquire(struct completion *x) {} 
static inline void complete_release(struct completion *x) {}



#define COMPLETION_INITIALIZER(work) \

	{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }

	{ 0, __SWAIT_QUEUE_HEAD_INITIALIZER((work).wait) }



#define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \

	(*({ init_completion_map(&(work), &(map)); &(work); }))
@@ -85,7 +85,7 @@ static inline void complete_release(struct completion *x) {} 
static inline void __init_completion(struct completion *x)

{

	x->done = 0;

	init_waitqueue_head(&x->wait);

	init_swait_queue_head(&x->wait);

}



/**

kernel/sched/completion.c

+19 −17
Original line number Diff line number Diff line
@@ -29,12 +29,12 @@ void complete(struct completion *x) 
{

	unsigned long flags;



	spin_lock_irqsave(&x->wait.lock, flags);

	raw_spin_lock_irqsave(&x->wait.lock, flags);



	if (x->done != UINT_MAX)

		x->done++;

	__wake_up_locked(&x->wait, TASK_NORMAL, 1);

	spin_unlock_irqrestore(&x->wait.lock, flags);

	swake_up_locked(&x->wait);

	raw_spin_unlock_irqrestore(&x->wait.lock, flags);

}

EXPORT_SYMBOL(complete);
@@ -58,10 +58,12 @@ void complete_all(struct completion *x) 
{

	unsigned long flags;



	spin_lock_irqsave(&x->wait.lock, flags);

	WARN_ON(irqs_disabled());



	raw_spin_lock_irqsave(&x->wait.lock, flags);

	x->done = UINT_MAX;

	__wake_up_locked(&x->wait, TASK_NORMAL, 0);

	spin_unlock_irqrestore(&x->wait.lock, flags);

	swake_up_all_locked(&x->wait);

	raw_spin_unlock_irqrestore(&x->wait.lock, flags);

}

EXPORT_SYMBOL(complete_all);
@@ -70,20 +72,20 @@ do_wait_for_common(struct completion *x, 
		   long (*action)(long), long timeout, int state)

{

	if (!x->done) {

		DECLARE_WAITQUEUE(wait, current);

		DECLARE_SWAITQUEUE(wait);



		__add_wait_queue_entry_tail_exclusive(&x->wait, &wait);

		do {

			if (signal_pending_state(state, current)) {

				timeout = -ERESTARTSYS;

				break;

			}

			__prepare_to_swait(&x->wait, &wait);

			__set_current_state(state);

			spin_unlock_irq(&x->wait.lock);

			raw_spin_unlock_irq(&x->wait.lock);

			timeout = action(timeout);

			spin_lock_irq(&x->wait.lock);

			raw_spin_lock_irq(&x->wait.lock);

		} while (!x->done && timeout);

		__remove_wait_queue(&x->wait, &wait);

		__finish_swait(&x->wait, &wait);

		if (!x->done)

			return timeout;

	}
@@ -100,9 +102,9 @@ __wait_for_common(struct completion *x, 


	complete_acquire(x);



	spin_lock_irq(&x->wait.lock);

	raw_spin_lock_irq(&x->wait.lock);

	timeout = do_wait_for_common(x, action, timeout, state);

	spin_unlock_irq(&x->wait.lock);

	raw_spin_unlock_irq(&x->wait.lock);



	complete_release(x);
@@ -291,12 +293,12 @@ bool try_wait_for_completion(struct completion *x) 
	if (!READ_ONCE(x->done))

		return false;



	spin_lock_irqsave(&x->wait.lock, flags);

	raw_spin_lock_irqsave(&x->wait.lock, flags);

	if (!x->done)

		ret = false;

	else if (x->done != UINT_MAX)

		x->done--;

	spin_unlock_irqrestore(&x->wait.lock, flags);

	raw_spin_unlock_irqrestore(&x->wait.lock, flags);

	return ret;

}

EXPORT_SYMBOL(try_wait_for_completion);
@@ -322,8 +324,8 @@ bool completion_done(struct completion *x) 
	 * otherwise we can end up freeing the completion before complete()

	 * is done referencing it.

	 */

	spin_lock_irqsave(&x->wait.lock, flags);

	spin_unlock_irqrestore(&x->wait.lock, flags);

	raw_spin_lock_irqsave(&x->wait.lock, flags);

	raw_spin_unlock_irqrestore(&x->wait.lock, flags);

	return true;

}

EXPORT_SYMBOL(completion_done);

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