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

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 5719176..234177d 100644 (file)
--- 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 *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);

diff --git a/include/linux/completion.h b/include/linux/completion.h
index 519e949..bf8e770 100644 (file)
--- 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(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);
 }
 
 /**

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index a1ad5b7..f15e961 100644 (file)
--- 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 @@ __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);