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From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
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	Davidlohr Bueso <dave@stgolabs.net>,
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	linuxppc-dev@lists.ozlabs.org,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: [patch V3 16/20] completion: Use simple wait queues
Date: Sat, 21 Mar 2020 12:26:00 +0100
Message-ID: <20200321113242.317954042@linutronix.de> (raw)
In-Reply-To: <20200321112544.878032781@linutronix.de>

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);



  parent reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-21 11:25 [patch V3 00/20] Lock ordering documentation and annotation for lockdep Thomas Gleixner
2020-03-21 11:25 ` [patch V3 01/20] PCI/switchtec: Fix init_completion race condition with poll_wait() Thomas Gleixner
2020-03-21 11:25 ` [patch V3 02/20] pci/switchtec: Replace completion wait queue usage for poll Thomas Gleixner
2020-03-21 11:25 ` [patch V3 03/20] usb: gadget: Use completion interface instead of open coding it Thomas Gleixner
2020-03-25  8:37   ` Felipe Balbi
2020-03-27 12:14     ` Sebastian Siewior
2020-03-21 11:25 ` [patch V3 04/20] orinoco_usb: Use the regular completion interfaces Thomas Gleixner
2020-03-21 11:25 ` [patch V3 05/20] acpi: Remove header dependency Thomas Gleixner
2020-03-21 11:25 ` [patch V3 06/20] nds32: Remove mm.h from asm/uaccess.h Thomas Gleixner
2020-03-21 11:25 ` [patch V3 07/20] csky: " Thomas Gleixner
2020-03-21 11:25 ` [patch V3 08/20] hexagon: " Thomas Gleixner
2020-03-23 21:46   ` Brian Cain
2020-03-21 11:25 ` [patch V3 09/20] ia64: " Thomas Gleixner
2020-03-21 11:25 ` [patch V3 10/20] microblaze: " Thomas Gleixner
2020-03-21 11:25 ` [patch V3 11/20] rcuwait: Add @state argument to rcuwait_wait_event() Thomas Gleixner
2020-03-21 11:25 ` [patch V3 12/20] powerpc/ps3: Convert half completion to rcuwait Thomas Gleixner
2020-03-21 13:22   ` Thomas Gleixner
2020-03-27 19:14   ` Geoff Levand
2020-03-21 11:25 ` [patch V3 13/20] Documentation: Add lock ordering and nesting documentation Thomas Gleixner
2020-03-23  2:55   ` Paul E. McKenney
2020-03-24 23:13     ` Thomas Gleixner
2020-03-25  0:28       ` Paul E. McKenney
2020-03-25 12:27         ` Documentation/locking/locktypes: Further clarifications and wordsmithing Thomas Gleixner
2020-03-25 16:02           ` Sebastian Siewior
2020-03-25 16:39             ` Paul E. McKenney
2020-03-25 16:54               ` Sebastian Siewior
2020-03-25 16:58           ` [PATCH v2] Documentation/locking/locktypes: minor copy editor fixes Randy Dunlap
2020-03-26  2:40             ` Paul E. McKenney
2020-03-21 11:25 ` [patch V3 14/20] timekeeping: Split jiffies seqlock Thomas Gleixner
2020-03-21 11:25 ` [patch V3 15/20] sched/swait: Prepare usage in completions Thomas Gleixner
2020-03-21 11:26 ` Thomas Gleixner [this message]
2020-03-23 15:20   ` [PATCH] completion: Use lockdep_assert_RT_in_threaded_ctx() in complete_all() Sebastian Siewior
2020-03-21 11:26 ` [patch V3 17/20] lockdep: Introduce wait-type checks Thomas Gleixner
2020-03-21 11:26 ` [patch V3 18/20] lockdep: Add hrtimer context tracing bits Thomas Gleixner
2020-03-21 11:26 ` [patch V3 19/20] lockdep: Annotate irq_work Thomas Gleixner
2020-03-21 11:26 ` [patch V3 20/20] lockdep: Add posixtimer context tracing bits Thomas Gleixner
2020-03-21 17:19 ` [patch V3 00/20] Lock ordering documentation and annotation for lockdep Davidlohr Bueso
2020-03-21 17:45   ` Thomas Gleixner

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