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Date:   Thu, 9 May 2019 14:31:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] RFC: console: hack up console_lock more v3
Message-ID: <20190509123104.GQ2589@hirez.programming.kicks-ass.net>
References: <20190509120903.28939-1-daniel.vetter@ffwll.ch>
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On Thu, May 09, 2019 at 02:09:03PM +0200, Daniel Vetter wrote:
> Fix this by creating a prinkt_safe_up() which calls wake_up_process
> outside of the spinlock. This isn't correct in full generality, but
> good enough for console_lock:
> 
> - console_lock doesn't use interruptible or killable or timeout down()
>   calls, hence an up() is the only thing that can wake up a process.

Wrong :/ Any task can be woken at any random time. We must, at all
times, assume spurious wakeups will happen.

> +void printk_safe_up(struct semaphore *sem)
> +{
> +	unsigned long flags;
> +	struct semaphore_waiter *waiter = NULL;
> +
> +	raw_spin_lock_irqsave(&sem->lock, flags);
> +	if (likely(list_empty(&sem->wait_list))) {
> +		sem->count++;
> +	} else {
> +		waiter = list_first_entry(&sem->wait_list,
> +					  struct semaphore_waiter, list);
> +		list_del(&waiter->list);
> +		waiter->up = true;
> +	}
> +	raw_spin_unlock_irqrestore(&sem->lock, flags);
> +
> +	if (waiter) /* protected by being sole wake source */
> +		wake_up_process(waiter->task);
> +}
> +EXPORT_SYMBOL(printk_safe_up);

Since its only used from printk, that EXPORT really isn't needed.

Something like the below might work.

---
 kernel/locking/semaphore.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 561acdd39960..ac0a67e95aac 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -38,7 +38,6 @@ static noinline void __down(struct semaphore *sem);
 static noinline int __down_interruptible(struct semaphore *sem);
 static noinline int __down_killable(struct semaphore *sem);
 static noinline int __down_timeout(struct semaphore *sem, long timeout);
-static noinline void __up(struct semaphore *sem);
 
 /**
  * down - acquire the semaphore
@@ -178,14 +177,24 @@ EXPORT_SYMBOL(down_timeout);
  */
 void up(struct semaphore *sem)
 {
+	struct semaphore_waiter *waiter;
+	DEFINE_WAKE_Q(wake_q);
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&sem->lock, flags);
-	if (likely(list_empty(&sem->wait_list)))
+	if (likely(list_empty(&sem->wait_list))) {
 		sem->count++;
-	else
-		__up(sem);
+		goto unlock;
+	}
+
+	waiter = list_first_entry(&sem->wait_list, struct semaphore_waiter, list);
+	list_del(&waiter->list);
+	waiter->up = true;
+	wake_q_add(&wake_q, waiter->task);
+unlock:
 	raw_spin_unlock_irqrestore(&sem->lock, flags);
+
+	wake_up_q(&wake_q);
 }
 EXPORT_SYMBOL(up);
 
@@ -252,12 +261,3 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
 {
 	return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
 }
-
-static noinline void __sched __up(struct semaphore *sem)
-{
-	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
-						struct semaphore_waiter, list);
-	list_del(&waiter->list);
-	waiter->up = true;
-	wake_up_process(waiter->task);
-}