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To:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>
From:   Chris Wilson <chris@chris-wilson.co.uk>
In-Reply-To: <20190506074553.21464-1-daniel.vetter@ffwll.ch>
Cc:     Petr Mladek <pmladek@suse.com>,
        John Ogness <john.ogness@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>
References: <20190502141643.21080-1-daniel.vetter@ffwll.ch>
 <20190506074553.21464-1-daniel.vetter@ffwll.ch>
Message-ID: <155739797736.28545.2942646931608459049@skylake-alporthouse-com>
User-Agent: alot/0.6
Subject: Re: [Intel-gfx] [PATCH] RFC: console: hack up console_lock more v2
Date:   Thu, 09 May 2019 11:32:57 +0100
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Quoting Daniel Vetter (2019-05-06 08:45:53)
> +/**
> + * printk_safe_up - release the semaphore in console_unlock
> + * @sem: the semaphore to release
> + *
> + * Release the semaphore.  Unlike mutexes, up() may be called from any
> + * context and even by tasks which have never called down().
> + *
> + * NOTE: This is a special version of up() for console_unlock only. It is only
> + * safe if there are no killable, interruptible or timing out down() calls.
> + */
> +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)
> +               wake_up_process(waiter->task);

From comparing against __down_common() there's a risk here that as soon
as waiter->up == true, the waiter may complete and make the onstack
struct semaphore_waiter invalid. If you store waiter->task locally under
the spinlock that problem is resolved.

Then there is the issue of an unprotected dereference of the task in
wake_up_process() -- I think you can wrap this function with
rcu_read_lock() to keep that safe, and wake_up_process() should be a
no-op if it races against process termination.
-Chris