[patch V3 16/20] completion: Use simple wait queues

Thomas Gleixner
2020-03-21T11:35:34Z

From: Thomas Gleixner <tglx@linutronix.de>

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>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
---
V2: Split out the orinoco and usb gadget parts and amended change log
---
drivers/usb/gadget/function/f_fs.c | 2 +-
include/linux/completion.h | 8 ++++----
kernel/sched/completion.c | 36 +++++++++++++++++++-----------------
3 files changed, 24 insertions(+), 22 deletions(-)

--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -1703,7 +1703,7 @@ static void ffs_data_put(struct ffs_data
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);
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -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(stru
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(stru
static inline void __init_completion(struct completion *x)
{
x->done = 0;
- init_waitqueue_head(&x->wait);
+ init_swait_queue_head(&x->wait);
}

/**
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -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 @@ static inline long __sched

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 comp
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 *
* 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);