On Thu 2019-05-09 22:06:33, Daniel Vetter wrote:
> console_trylock, called from within printk, can be called from pretty
> much anywhere. Including try_to_wake_up. Note that this isn't common,
> usually the box is in pretty bad shape at that point already. But it
> really doesn't help when then lockdep jumps in and spams the logs,
> potentially obscuring the real backtrace we're really interested in.
> One case I've seen (slightly simplified backtrace):
>
> Fix this specific locking recursion by moving the wake_up_process out
> from under the semaphore.lock spinlock, using wake_q as recommended by
> Peter Zijlstra.
It might make sense to mention also the optimization effect mentioned
by Peter.
> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index 561acdd39960..7a6f33715688 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -169,6 +169,14 @@ int down_timeout(struct semaphore *sem, long timeout)
> }
> EXPORT_SYMBOL(down_timeout);
>
> +/* Functions for the contended case */
> +
> +struct semaphore_waiter {
> + struct list_head list;
> + struct task_struct *task;
> + bool up;
> +};
> +
> /**
> * up - release the semaphore
> * @sem: the semaphore to release
> @@ -179,24 +187,25 @@ EXPORT_SYMBOL(down_timeout);
> void up(struct semaphore *sem)
> {
> unsigned long flags;
> + struct semaphore_waiter *waiter;
> + DEFINE_WAKE_Q(wake_q);
We need to call wake_q_init(&wake_q) to make sure that
it is empty.
Best Regards,
Petr
> 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);
> + } else {
> + 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);
> + }
> raw_spin_unlock_irqrestore(&sem->lock, flags);
> +
> + wake_up_q(&wake_q);
> }
> EXPORT_SYMBOL(up);
>
> -/* Functions for the contended case */
> -
> -struct semaphore_waiter {
> - struct list_head list;
> - struct task_struct *task;
> - bool up;
> -};
> -
> /*
> * Because this function is inlined, the 'state' parameter will be
> * constant, and thus optimised away by the compiler. Likewise the
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