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Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock()

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On Tue, 7 Mar 2017, Peter Zijlstra wrote:

> On Tue, Mar 07, 2017 at 03:18:46PM +0100, Thomas Gleixner wrote:
> > On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> > > +/**
> > > + * rt_mutex_cleanup_proxy_lock() - Cleanup failed lock acquisition
> > > + * @lock:		the rt_mutex we were woken on
> > > + * @waiter:		the pre-initialized rt_mutex_waiter
> > > + *
> > > + * Clean up the failed lock acquisition as per rt_mutex_wait_proxy_lock().
> > > + *
> > > + * Returns:
> > > + *  true  - did the cleanup, we done.
> > > + *  false - we acquired the lock after rt_mutex_wait_proxy_lock() returned,
> > > + *          caller should disregards its return value.
> > 
> > Hmm. How would that happen? Magic owner assignement to a non waiter? The
> > callsite only calls here in the failed case.
> 
> Ah, but until the remove_waiter() below, we _still_ are a waiter, and
> thus can get assigned ownership.
> 
> > > + *
> > > + * Special API call for PI-futex support
> > > + */
> > > +bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
> > > +				 struct rt_mutex_waiter *waiter)
> > > +{
> > > +	bool cleanup = false;
> > > +
> > > +	raw_spin_lock_irq(&lock->wait_lock);
> > > +	/*
> > > +	 * If we acquired the lock, no cleanup required.
> > > +	 */
> > > +	if (rt_mutex_owner(lock) != current) {
> > > +		remove_waiter(lock, waiter);
> 
> See, up till this point, we still a waiter and any unlock can see us
> being one.

Hmm, true. So the comments should explain that

Thanks,

	tglx

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[PATCH -v5 00/14] the saga of FUTEX_UNLOCK_PI wobbles continues Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
  [PATCH -v5 02/14] futex: Add missing error handling to FUTEX_REQUEUE_PI Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
  [PATCH -v5 04/14] futex: Use smp_store_release() in mark_wake_futex() Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
  [PATCH -v5 10/14] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
    Re: [PATCH -v5 10/14] futex: Pull rt_mutex_futex_unlock() out from  under hb->lock Thomas Gleixner <tglx@linutronix.de> - 2017-03-07 19:20 +0100
      Re: [PATCH -v5 10/14] futex: Pull rt_mutex_futex_unlock() out from  under hb->lock Peter Zijlstra <peterz@infradead.org> - 2017-03-07 19:50 +0100
  [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
    Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Thomas Gleixner <tglx@linutronix.de> - 2017-03-07 15:40 +0100
      Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Peter Zijlstra <peterz@infradead.org> - 2017-03-07 19:50 +0100
      Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Peter Zijlstra <peterz@infradead.org> - 2017-03-13 10:30 +0100
        Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Thomas Gleixner <tglx@linutronix.de> - 2017-03-13 15:30 +0100
          Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism Peter Zijlstra <peterz@infradead.org> - 2017-03-13 16:20 +0100
  [PATCH -v5 11/14] futex,rt_mutex: Introduce rt_mutex_init_waiter() Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
  [PATCH -v5 07/14] futex: Change locking rules Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:10 +0100
    Re: [PATCH -v5 07/14] futex: Change locking rules Thomas Gleixner <tglx@linutronix.de> - 2017-03-07 14:50 +0100
      Re: [PATCH -v5 07/14] futex: Change locking rules Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-03-07 18:20 +0100
        Re: [PATCH -v5 07/14] futex: Change locking rules Peter Zijlstra <peterz@infradead.org> - 2017-03-07 19:10 +0100
  [PATCH -v5 06/14] futex,rt_mutex: Provide futex specific rt_mutex API Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:50 +0100
  [PATCH -v5 05/14] futex: Remove rt_mutex_deadlock_account_*() Peter Zijlstra <peterz@infradead.org> - 2017-03-04 11:50 +0100
  [PATCH -v5 08/14] futex: Cleanup refcounting Peter Zijlstra <peterz@infradead.org> - 2017-03-04 12:50 +0100
  [PATCH -v5 03/14] futex: Cleanup variable names for futex_top_waiter() Peter Zijlstra <peterz@infradead.org> - 2017-03-04 12:50 +0100
  [PATCH -v5 12/14] futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock() Peter Zijlstra <peterz@infradead.org> - 2017-03-04 12:50 +0100
    Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure  rt_mutex_finish_proxy_lock() Thomas Gleixner <tglx@linutronix.de> - 2017-03-07 15:50 +0100
      Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure  rt_mutex_finish_proxy_lock() Thomas Gleixner <tglx@linutronix.de> - 2017-03-07 19:50 +0100
      Re: [PATCH -v5 12/14] futex,rt_mutex: Restructure  rt_mutex_finish_proxy_lock() Peter Zijlstra <peterz@infradead.org> - 2017-03-07 19:50 +0100
    [PATCH] futex: move debug_rt_mutex_free_waiter() further down Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-03-08 16:40 +0100
      Re: [PATCH] futex: move debug_rt_mutex_free_waiter() further down Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-03-08 16:40 +0100
        Re: [PATCH] futex: move debug_rt_mutex_free_waiter() further down Steven Rostedt <rostedt@goodmis.org> - 2017-03-08 17:40 +0100
      Re: [PATCH] futex: move debug_rt_mutex_free_waiter() further down Steven Rostedt <rostedt@goodmis.org> - 2017-03-08 17:30 +0100
      Re: [PATCH] futex: move debug_rt_mutex_free_waiter() further down Peter Zijlstra <peterz@infradead.org> - 2017-03-13 10:20 +0100

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