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Date:   Thu, 9 May 2019 15:05:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Chris Wilson <chris@chris-wilson.co.uk>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Petr Mladek <pmladek@suse.com>,
        John Ogness <john.ogness@linutronix.de>,
        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>
Subject: Re: [Intel-gfx] [PATCH] RFC: console: hack up console_lock more v2
Message-ID: <20190509130504.GW2623@hirez.programming.kicks-ass.net>
References: <20190502141643.21080-1-daniel.vetter@ffwll.ch>
 <20190506074553.21464-1-daniel.vetter@ffwll.ch>
 <155739797736.28545.2942646931608459049@skylake-alporthouse-com>
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On Thu, May 09, 2019 at 11:32:57AM +0100, Chris Wilson wrote:
> 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.

task_struct is not RCU protected, see task_rcu_dereference() for magic.