From: Petr Mladek <pmladek@suse.com>
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>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.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] kernel/locking/semaphore: use wake_q in up()
Date: Fri, 10 May 2019 11:28:19 +0200 [thread overview]
Message-ID: <20190510092819.elu4b7fcojzcek2q@pathway.suse.cz> (raw)
In-Reply-To: <20190509200633.19678-1-daniel.vetter@ffwll.ch>
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
next prev parent reply other threads:[~2019-05-10 9:28 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-09 12:09 [PATCH] RFC: console: hack up console_lock more v3 Daniel Vetter
2019-05-09 12:21 ` Chris Wilson
2019-05-09 12:31 ` Peter Zijlstra
2019-05-09 13:06 ` Daniel Vetter
2019-05-09 13:36 ` Peter Zijlstra
2019-05-09 14:56 ` Petr Mladek
2019-05-09 16:43 ` Daniel Vetter
2019-05-10 9:15 ` Petr Mladek
2019-05-10 9:51 ` Sergey Senozhatsky
2019-05-10 15:05 ` Daniel Vetter
2019-05-09 20:06 ` [PATCH] kernel/locking/semaphore: use wake_q in up() Daniel Vetter
2019-05-10 5:50 ` Sergey Senozhatsky
2019-05-10 7:51 ` Daniel Vetter
2019-05-10 9:28 ` Petr Mladek [this message]
2019-05-10 15:20 ` Daniel Vetter
2019-05-15 11:53 ` Petr Mladek
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